I see a repeated assumption that puzzles me. From another well asked and answered question:

On one hand it is impossible for the UK to leave the Single Market while maintaining an open border with an EU member

The comments and answers point out that both the Irish and the UK do not want a hard border, so why is it impossible to leave the Single Market without one? I presume the EU cannot legally force a country that is not a member to enforce its border with them and it can't force the UK to remain an EU member if it decides on a soft border.

This question also includes the assumption and has an answer titled "They can't" but the reasons it gives aren't things that create impossibilities. For example, it claims:

any Brexit implies stricter borders

Maybe for Ireland/EU but the UK does not have to impose any border restriction. It seems to be assumed, and yet it's been a soft border for almost 100 years:

the border is essentially open, allowing free passage of people since 1923 and of goods since 1993

I'm looking for something of the ilk of a technical/legal reason, not one that is about negotiating stances or consequences or political inconveniences that go "against Brexit". I'll give 2 examples of things I'm not looking for and why because I want this to be a very strict question.

  1. Negotiating stance: The EU demands it or it refuses to sign a trade deal. That doesn't make it impossible even though the UK could concede to it, the UK could simply ignore it.
  2. Consequence: (From this answer) The UK would have to accept the free flow of EU citizens over the border. If foreigners can master the Ulster accent well enough to hide illegally then good for them! ;-) But, this also doesn't make it impossible.

It's an assumption so often made and rarely challenged that I'm very curious to find the answer. It appears to be the EU's problem entirely as it would threaten their customs union and that they are the ones that require a hard border after Brexit. However, maybe I missed something obvious or there's an international law I'm unaware of that makes it so?

  • 11
    By definition if the UK leaves the single market it'll end up with a border. The question revolves around how hard that border will be. At one end the UK is adamant about refusing freedom of movement and ECJ jurisdiction - i.e. it'll necessarily end up being a hard border. At another respecting the Good Friday agreement mandates a soft border in Ireland. And yet another the UK wants no border in the Irish Sea. Having all three at the same time is unlikely if not possible. Commented Mar 4, 2018 at 7:44
  • @DenisdeBernardy How would refusing ECJ jurisdiction affect the hardness of the border?
    – ian
    Commented Mar 4, 2018 at 16:22
  • 4
    @iain: Because the Council, the EC, and the EP all made it clear that ECJ jurisdiction was a sine qua none condition to getting a soft border. Commented Mar 4, 2018 at 16:46
  • 1
    "I presume the EU cannot legally force a country that is not a member to enforce its border with them" – While true, the EU can just enforce the border on its side of the border.
    – Daniel
    Commented Aug 9, 2019 at 22:43
  • @Daniel That could be true, but as Ireland's Taoiseach has repeatedly said (in the Irish parliament and beyond) Ireland will not put up a hard border. I'm wondering how the EU (as a separate institution to the Irish government) would do it in those circumstances?
    – ian
    Commented Aug 10, 2019 at 8:23

8 Answers 8


In the strictest sense, it is of course possible; but it doesn’t make any sense, unless Brexit is only for symbolism.

The whole point of Brexit (beside the symbolism) and of leaving the single market and the customs union is to allow Britain to act independently from its former partners – at least in trade issues. On the other hand, one main point of a single market is to allow goods to be traded as seemlessly between, say, Madrid and London as they traditionally are traded between Manchester and London; there should be no trade barriers in a single market.

So let’s consider some scenarios where Brexit Britain starts doing its own thing while the Irish border remains fully open:

  • The British parliament and government decide, in order to protect the British strawberry growers from unfair (sunnier) competition, to introduce a 200% import duty on strawberries. (This is a tariff barrier.) The meaning is that strawberries bought for £ 2 a kilogram in Spain should cost at least £ 6 when reaching England. However, importers take their Spanish strawberries to Dublin (no import duty because single market), then across the Irish border to Belfast (no import duty because fully open border), then to England (no import duty because of the UK’s own internal single market), and sell for £ 3. The British strawberry growers and the government aren’t happy.

  • The British authorities decide to allow treating chickens with chlorine. This is not allowed in the EU, and chlorinated chickens aren’t supposed to be in circulation within the EU. (This new regulatory divergence between Brexit Britain and the EU amounts to a non-tariff barrier.) However, Northern Irish farmers take their chlorinated chickens to the Republic (possible because fully open border) and sell them to supermarkets there, and in other EU members. The EU and its remaining member states aren’t happy.

  • The president of the US, Donald Trump, likes Brexit Britain so much that he generously exempts it from his 25% import duty on steel. Belgian steel works transport their produce via Ireland and Northern Ireland to the US, avoiding the duty that is supposed to target them. President Trump isn’t happy.

Of course, Brexit Britain could refrain from utilizing its newly-found power to diverge, voluntarily keeping the rules that the EU has. That, however, would mean “a Brexit in name only”, i.e. pure symbolism.

  • 82
    And despite this being incredibly simple to understand, and having been repeatedly spelled out to the British public (both before the referendum and since), plenty of people still think it’s all just lies and somehow the realities of global trade can somehow be magicked away.
    – eggyal
    Commented Mar 4, 2018 at 15:30
  • 22
    @iain: It may be true that Britain doesn’t care about the EU’s happiness, but I gave three examples specifically to show that Britain and the EU and even third parties may be negatively affected. (And the third party matters to Britain, too, because it will react like this: “If you can’t stop our generous trade agreement being exploited by EU traders, we’ll need to repeal it.”)
    – chirlu
    Commented Mar 4, 2018 at 16:56
  • 37
    "There's nothing to stop the British authorities putting in place passport checks at the seaports in the western UK." <--- and thus you make a hard border in the Irish Sea :) This particular configuration leads to NI feeling like a second class citizen within the UK and triggers complications further down the road. In the short term, the DUP will say hell no and May will need to find more votes from Labour to get things passed.
    – Kaithar
    Commented Mar 5, 2018 at 7:05
  • 30
    Regarding putting passport checks in the Irish Sea (i.e., from NI to Mainland UK) - It's worth pointing out that the people who would be vehemently opposed to this (the NI Unionists) are currently the only thing keeping Theresa May's government in office (by forming a coalition with the minority Tory party). Good luck with riling them... parliament.uk/mps-lords-and-offices/mps/… Commented Mar 5, 2018 at 9:30
  • 26
    As for passport controls: that presumes everybody has a passport or an ID card, which is not the case in the UK. It's perfectly possible not to have a passport, and if that would exclude one from travelling in one's own country, that would not be popular. Commented Mar 5, 2018 at 15:37

The EU has customs at its borders. This allows it to, for example, collect tariffs on imports from the USA. So suppose the EU sets a 10% tariff on US cars. Now when a car moves from the US to Belfast, the UK collects a tariff. The UK can then re-export the car to Dublin, and there is no tariff because the UK and Ireland (the republic) are in a customs union.

Now suppose the UK leaves the customs union, and negotiates a trade deal with the US, which allows for free trade in cars between the UK and the US. If the Irish border remains completely open, then a US exporter could bypass EU tariffs by exporting to the UK and then re-exporting to the EU through Ireland. It would make a hole in the EU customs border.

International trade is a big enough deal that just "not enforcing" is not an option. Either Northern Ireland remains in a customs union, or the EU would have to impose some form of customs control at the border.

There has to be a customs border somewhere. The question is where:

  • In the Atlantic? (Not acceptable to UK government)
  • Between Northern Ireland and the Republic? (Breaks Good Friday Agreement; not acceptable to Ireland)
  • In the Irish sea? (Breaks Good Friday Agreement; absolutely unacceptable to the UK government)
  • In the English Channel? (Ireland leaves the EU, absolutely unacceptable to Ireland)

Every option breaks someone's red lines.

  • 1
    Could add that a border in the Irish Sea is not only unacceptable to the UK government, but also violates the Good Friday Agreement.
    – gerrit
    Commented Mar 4, 2018 at 14:30
  • 23
    @iain borders are bidirectional. The UK can say what it likes about exporting to the EU, but the EU also has a say in what it imports from the UK. If the UK has a better trade deal with another country, that country might find better profit by exporting to the UK then taking it across the non-border to Eire. That's going to deprive EU states of import taxes. And if the UK doesn't have better trade deals, what's the point in "leaving so we can negotiated our own deals"?
    – Kaithar
    Commented Mar 4, 2018 at 19:07
  • 2
    @JamesK That is simply not factually correct.
    – ian
    Commented Mar 4, 2018 at 23:40
  • 3
    @iain There is more to life than money, but trade deals is one of the major rallying cries of the Leavers... that we'll be able to negotiate better trade deals for ourselves once we're out of the EU.
    – Kaithar
    Commented Mar 5, 2018 at 7:07
  • 1
    @phoog I'm not sure. I've read it somewhere. I could be wrong.
    – gerrit
    Commented Dec 23, 2018 at 21:35

I'm looking for something of the ilk of a technical/legal reason, not one that is about negotiating stances or consequences or political inconveniences that go "against Brexit". I'll give 2 examples of things I'm not looking for and why because I want this to be a very strict question.

  1. Negotiating stance: The EU demands it or it refuses to sign a trade deal. That doesn't make it impossible even though the UK could concede to it, the UK could simply ignore it.

Actually, there is a technical/legal reason why that's backwards. If there is no trade deal, World Trade Organisation rules effectively require there to be a hard border. I suppose you could say that it's still possible because both the UK and the EU could leave the WTO or drop all tariffs on imports, but that would be deep into the ridiculous.

The issue is that the WTO restricts discrimination. The first principle of the WTO trading system is

Most-favoured-nation (MFN): treating other people equally

Under the WTO agreements, countries cannot normally discriminate between their trading partners. Grant someone a special favour (such as a lower customs duty rate for one of their products) and you have to do the same for all other WTO members.


Some exceptions are allowed. For example, countries can set up a free trade agreement that applies only to goods traded within the group — discriminating against goods from outside. Or they can give developing countries special access to their markets. Or a country can raise barriers against products that are considered to be traded unfairly from specific countries. And in services, countries are allowed, in limited circumstances, to discriminate. But the agreements only permit these exceptions under strict conditions. ...

On a narrow technical level, it would be possible for the UK to leave the Single Market and the Customs Union and have a customs union similar to the EU-Turkey one but more comprehensive: i.e. complete free movement of goods but not people or services. The obstacles to such a deal are more political and practical (lack of time) than legal, and any legal issues could in principle be fixed were there political will and time. Absent such a comprehensive trade deal, both parties will be required to levy tariffs (or drop tariffs for the entire world), which means having a mechanism to levy them, and that means having a border.

  • 5
    Additionally, this trade deal would probably likely be almost completely reliant on the UK's acceptance of rules made within the EU - politically unacceptable to many if not most brexiteers.
    – Miller86
    Commented Mar 5, 2018 at 16:16
  • 9
    For what it's worth, I'm not sure if you've seen global politics lately, but knee deep (or neck deep) in the ridiculous is sort of par for the course these days...
    – corsiKa
    Commented Mar 6, 2018 at 15:34
  • 4
    @trapper That's not a fair summary of the article. What it's saying is that the WTO won't actively enforce its rules, but will wait for another WTO member to make a complaint. Which will happen almost immediately, when other countries see Irish exporters to the UK getting more favourable treatment than their own countries' exporters.
    – Mike Scott
    Commented Feb 27, 2019 at 14:07
  • 3
    @trapper How can you charge tariffs without checking at the border to see what tariffs are due? You can have trusted trader schemes and pre-submission of paperwork, but without border checks you can't stop lorries crossing the border without having done that paperwork. The WTO rules don't say "there shall be a hard border", but it's hard to see how you can follow them without having one.
    – Mike Scott
    Commented Feb 27, 2019 at 14:39
  • 1
    @MikeScott I do agree it will be difficult to prevent all smuggling, but this is generally the case anyway. My point was just that WTO rules do not require a hard border, and there isn’t going to be one
    – trapper
    Commented Feb 27, 2019 at 14:48

One of Theresa May's red lines is that the UK must be able to do its own trade deals with other countries. This is not compatible with a soft border in Northern Ireland, because those trade deals (like all trade deals) are certain to include a requirement that the UK controls the import of goods to make sure that goods shipped from the UK to the country in question are genuinely from the UK and haven't slipped into the UK from a country not included in the trade deal, such as Ireland. Similarly, the EU's existing trade deals with other countries don't allow it to let goods from the UK into the EU unchecked.


After brexit both the EU and the UK will have its own borders. In principle you should not think of this as a single border but as one border exiting the EU and another border entering the UK or vice versa. This actually used to be the case between most countries with a part of no-mans land in between. Each country has full control over its own borders unless otherwise negotiated or in violation of international law.

For practicality both borders are usually combined into a single border.

In the case of a hard brexit with the UK refusing to close its border the EU (of course all presumed) will presumable impose a hard border on all incoming goods and people while not caring about the outgoing. So in this way the EU can impose a one-way hard border on the UK. The UK of course can do similar thing with its own border.

  • But couldn't the UK and the EU have a border and still be in the same single market? Commented Mar 5, 2018 at 13:40
  • 3
    Note that the NI border runs through the land and in some cases houses of quite a lot of people.
    – pjc50
    Commented Mar 5, 2018 at 15:30
  • 1
    @Trilarion If the EU and UK are in the same single market but not a customs union, there not only can be a border, there has to be. That's the situation with Norway and Switzerland, which both have a hard border with the EU (for goods; people just get waved through, since both countries are in Schengen). It requires both a single market and a customs union for there to be no border controls on goods.
    – Mike Scott
    Commented Feb 27, 2019 at 14:01

Having no hard borders is the very definition of the single market. If the UK leaves the single market, then it would require consent from both the UK and the EU not to implement a strict border. This mutual consent would take the form of a treaty, e.g. with Switzerland being a member of Schengen, yet not being an EU member.

So, in theory it would be possible for the UK to leave the single market and having no hard irish border, if the UK and the EU agree to do so. However, this would only work as long as the UK's economic regulations remained compatible to the EU's single market.

  • 3
    The Swiss model will not work because (1) it has a guillotine clause that forces the Swiss to accept EU workers, or else ALL deals are bust. The Swiss voted themselves immigration quota running foul of this and had to give in! (2) The EU doesn't like the model, so it's unlikely it will repeat it. Refs on request. Commented Jul 21, 2018 at 10:03

The EU can impose that Ireland maintains a hard border regardless of the UK's involvement. They can impose this to protect the trade agreements of the single market from either directly or indirectly benefiting the UK.

By contrast, a soft border cannot protect the trade agreements of the single market from benefiting the UK, nor can it protect against the exploitation of any of the UK's future trade agreements.

Edit: To reflect a direct answer to the question posed, it's not impossible by the literal definition of the word, there are no universal rules/laws when it comes to trade and borders between countries, everything can be negotiated and decided upon. The issue solely revolves around negotiation with the EU and can be solved if all parties come to agreement. The position is seen as practically impossible, a semantic use of the word to emphasise the extreme unlikeliness of any such agreement.

  • 2
    Please source the claim that the EU can impose 'hard borders', and quote the relevant section.
    – Chloe
    Commented Mar 5, 2018 at 4:56
  • 4
    @Chloe - No individual EU country is allowed to make its own trade deals - ukandeu.ac.uk/explainers/trade-deals-with-third-countries. A soft border between Ireland and NI could be seen as an individual trade deal for Ireland and as such is not allowed.
    – Qwerky
    Commented Mar 5, 2018 at 10:34
  • 1
    @Qwerky "A soft border between Ireland and NI could be seen as an individual trade deal for Ireland" - Any official word on that?
    – ian
    Commented Mar 6, 2018 at 4:15
  • 3
    @Chloe TFEU, Title V, Chapter 1, Article 67, Subsection 2: "It ... shall frame a common policy on asylum, immigration and external border control, based on solidarity between Member States, which is fair towards third-country nationals." -- The EU Member States collectively agree to the EU border.
    – Xpndable
    Commented Mar 6, 2018 at 9:12
  • 2
    @iain My point wasn't that they have to have a hard border, my point is it isn't the UK or Ireland's choice to make. As borders are bi-directional, and Ireland doesn't get to make the choice, both the UK and Ireland are still bound by what the other EU members decide and agree upon (and if no new agreement can be made, existing policy stands - a hard border). The burden falls to the UK to persuede the EU to assist Ireland in bargaining for a border policy, lest they be seen as doing nothing, and blame falls to Brexit as the cause of this border issue.
    – Xpndable
    Commented Mar 6, 2018 at 15:27

The EU cannot legally force a country that is not a member to enforce its border

True, but the Republic of Ireland is a member of the EU, so the EU can force them to enforce a border with the UK on their side. Even if the UK would let anyone in like before, would you call a border which can be crossed freely one way but not the other a "soft border"? Besides, it's very unlikely to happen in practice, because one of the key points of leaving the EU was for the UK to reclaim its right to reinforce their own borders.

  • Yes, I would call it a soft border because it's soft on the side that matters to the UK with respect to the claimed problem. If Ireland were forced to harden the border (I'd love to see that tried) then the EU would have to take the blame, something it does not want. As to the "right to reinforce their own borders", it's possible to want a right but not use it in every situation. You can want the right for free speech but never say anything controversial, the application is separate from the autonomy aspect.
    – ian
    Commented Apr 2, 2019 at 8:01

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .