I do not agree with the answer which says it is unclear whether it would be a political or a legal issue. Unless someone can provide evidence to the contrary, I believe that it is certainly a legal issue, and one that the European courts have jurisdiction over. Whether or not an Article 50 notice is revocable is uncertain, but whilst I personally see compelling reasons for it not being revocable, I haven't actually seen any reasoned arguments on the other side, other than politicians and other commentators giving their (unsubstantiated) opinions. If anyone has access to any concrete legal arguments for a notice being revocable, it would be great to post them as answers here.
The hierarchy of laws in the EU is essentially this:
- EU primary legislation: Treaties (TEU and TFEU)
- EU secondary legislation: Regulations, Directives, Decisions
- Member State law
A reasonable approximation to this exists in the USA where you have the Constitution (~EU primary legislation), federal law (~EU secondary legislation) and state law (~Member State law).
In general, each level in the hierarchy takes precedent over the levels below it, although the rules of enforcing EU law in a Member State are complicated (and I won't go into them here).
Article 50 is found in the TEU and as such is at the top of the hierarchy. The treaties define all aspects of how the EU operates and what powers it has. An EU institution such as the Council or the Commission exists, and has powers, because of the treaties. For that reason, unless the treaties specifically give jurisdiction to an EU institution to determine whether or not an Article 50 notice is revocable, then they do not have that jurisdiction, and it falls to the courts. They will make their decision in the same way that they make all decisions: by interpreting the law (in this case, the treaties). To do that they'll be assisted by various things such as previous cases, the purpose of the law (i.e. the intention of the drafters), principles of interpretation, etc.
The relevant paragraphs of Article 50 for our purposes are 1 - 3:
- Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements.
- A Member State which decides to withdraw shall notify the European Council of its intention. [rest of paragraph cut].
- The Treaties shall cease to apply to the State in question from the date of entry into force of the withdrawal agreement or, failing that, two years after the notification referred to in paragraph 2, unless the European Council, in agreement with the Member State concerned, unanimously decides to extend this period.
It has been argued in another answer that a future government could hypothetically repeal the European Union (Notification of Withdrawal) Act 2017 and that this would make the already issued notification fall foul of Article 50(1) as the "constitutional requirements" wouldn't be met. Paragraph 1 of that act states:
The Prime Minister may notify, under Article 50(2) of the Treaty on
European Union, the United Kingdom’s intention to withdraw from the
The Act received Royal Assent on 16th March 2017 and the UK notified the Council on 29th March 2017. Since the Act hadn't been repealed beforehand, the Prime Minister was acting lawfully when she submitted that notification. Repealing the Act would merely remove the power to notify from the Prime Minister going forward.
In any case, even if the Prime Minister's past notification could somehow be retrospectively made unlawful, it won't have an effect in EU law. Article 50(1) requires that the decision to withdraw be made "in accordance with [the Member State's] own constitutional requirements". Clearly that requirement was satisfied when the decision to withdraw was made. The actual notification itself is dealt with in 50(2) which says nothing about constitutional requirements.
Moving on, there is currently legal uncertainty as to whether or not a notice can be withdrawn, and there is plenty of legal opinion on both sides of the debate. We can't know for sure unless a case comes before the EU courts.
However, in my opinion, there are serious obstacles in the way of a notice being revocable. It is often said that Article 50 is "silent" on withdrawal of a notice. I do not believe this to be the case. I believe it very clearly rules out such a possibility.
Article 50(3) provides:
The Treaties shall cease to apply to the State in question from the
date of entry into force of the withdrawal agreement or, failing that,
two years after the notification referred to in paragraph 2,
unless the European Council, in agreement with the Member State concerned, unanimously decides to extend this period.
It seems clear to me that the word "unless" (emphasised by me above) was intended by the drafters to cover the circumstances under which the rule which comes before it can be ignored. There is a principle of statutory interpretation, "the express mention of one thing excludes all others" (in latin: Expressio unius est exclusio alterius). The drafters of Article 50 have provided exactly one method of avoiding EU withdrawal 2 years following notification (in the absence of a withdrawal agreement) after the word "unless": unanimous agreement of the Council to extend the period for reaching agreement. Or to put it another way, it unambiguously states that unless there is a unanimously agreed extension, then the Member State will no longer be in the EU either after 2 years from notification, or in accordance with the withdrawal agreement. If the drafters had intended for a notice to be revocable, then they had their chance to provide for this after the word "unless". They didn't, so we must assume that it wasn't intended for this to be possible.
Another compelling argument is the actual provision for extending the 2 year period in which agreement must be reached, which requires unanimous agreement of the other Member States via the Council. Allowing a notice to be withdrawn would essentially make a nonsense of this provision, as a Member State wanting to extend beyond 2 years could simply withdraw their notice and resubmit it the next day, thus gaining an additional 2 years without any agreement from the other Member States. It can't have been the drafters' intention to allow this requirement to be so easily circumvented.
I thought it worth mentioning another couple of possibilities:
The EU treaties could be modified to specifically allow the notice to be withdrawn. However, this would require unanimous agreement of all Member States and historically has been difficult to achieve, and slow when it has been achieved. I suppose one could imagine a scenario where the Council agrees to extend the agreement period using Article 50(3), to allow sufficient time for Treaty change.
The withdrawal agreement could be finalised simultaneously to an Article 49 agreement. Article 49 describes how a new state may join the EU. If the Article 49 agreement was worded to provide for the UK retaining the same membership terms as it has now, then the effect would be the same as if notice was withdrawn. The requirements for achieving an Article 49 agreement are even more stringent than for achieving an Article 50(3) extension: unanimous agreement of the Council, absolute majority of the European Parliament, and ratification by each Member State.