What are the UK's "constitutional requirements" for Article 50 notification of withdrawal from EU?

Question

Article 50 of the Lisbon Treaty says:

1. Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements.

What are the UK's constitutional requirements in this regard?

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Update 3 July 2016

London law firm Mishcon de Reya is preparing legal action to clarify this issue. Their press release says "Legal steps have been taken to ensure the UK Government will not trigger the procedure for withdrawal from the EU without an Act of Parliament."

Blogger Jack of Kent surmises that they are seeking a "Declaration":

A good guess that it is for a “Declaration” on what Article 50(1) requires as a matter of English (and Welsh) law. A “Declaration” is a discretionary remedy of the courts ... when the correct legal position on something ... needs to be established.

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An earlier-filed case has emerged: http://www.theguardian.com/politics/2016/jul/05/deadline-approaches-government-response-brexit-legal-challenge-article-50

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More on this issue: http://www.independent.co.uk/news/uk/politics/in-full-the-letter-from-1000-lawyers-to-david-cameron-over-eu-referendum-brexit-legality-a7130226.html

Answer 1

In the UK, parliament is sovereign. Parliament has exercised its power by passing a bill to hold a referendum. The result of the referendum now gives the government the authority to act to leave the EU.

The constitution of the UK, based as it is on statute law, common law and tradition, gives final authority in constitutional matters to parliament. There is a (quite recent) tradition that in matters of significant change to the constitution, the people should be consulted, generally by referendum.

However if authorised by parliament or by a referendum, the decision to invoke article 50 is a matter for the executive, Ie the cabinet and the prime minister. This power follows from the general ability of the executive to form treaties and enact foreign policy, and derives ultimately from the powers of the monarch.

Answer 2

Today's decision in the High Court is that the government does not have the power to trigger Article 50 and withdraw the UK from the EU without the consent of Parliament.

R (Miller) -V- Secretary of State for Exiting the European Union [2016] EWHC 2768 (Admin)

The court does not accept the argument put forward by the Government. There is nothing in the text of the 1972 Act to support it.

In the judgment of the court the argument is contrary both to the language used by Parliament in the 1972 Act and to the fundamental constitutional principles of the sovereignty of Parliament and the absence of any entitlement on the part of the Crown to change domestic law by the exercise of its prerogative powers. The court expressly accepts the principal argument of the claimants.

For the reasons set out in the judgment, we decide that the Government does not have power under the Crown’s prerogative to give notice pursuant to Article 50 for the UK to withdraw from the European Union.

Summary: R (Miller) -V- Secretary of State for Exiting the European Union

An appeal to the Supreme Court is expected.

Answer 3

Well for a start the UK doesn't have a 'constitution' as such. The result of the referendum is not legally binding in terms of whether it is enacted upon or not. However, the EU are now looking to trigger Article 50 on our behalf because at earliest it doesn't look like it would be set in motion until October/November and this doesn't sit well with the EU. It would set a bad precedent where a country may then do a similar action in order to hold the EU to ransom regarding trade agreements and so forth.