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