The Article 50 Challenge (Twitter hashtag #A50Challenge & @A50Challenge) is a case filed against the Government on 22nd December 2017 in the High Court of England and Wales and has received a response from the Government yesterday. Updates to their case are on their website linked to above.
The basis of this challenge is the text of Article 50 of the Treaty on European Union.
Referendums have never had any legal force in the UK, except when expressly so provided by Parliament. Indeed, there have only been three UK-wide referendums so far, with the first being about entering the EEC (as then) and the third being about exitting the EU (as now) - the second was the one on the voting system in May 2011, which was the only one in which it was expressly provided by Act of Parliament that the Government would implement its results.Article 50 wrote:1. Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements.
Following the Gina Miller judgment of the Supreme Court, Parliament passed the European Union (Notification of Withdrawal) Act 2017 to confer power on the Prime Minister to notify the EU under the terms of Article 50.
However, there was never a decision taken by Parliament to actually trigger Article 50, by either a resolution or by Act of Parliament. It is unclear under the UK's uncodified constitution whether the Prime Minister can unilaterally trigger Article 50 or whether Parliamentary authorisation of the Prime Minister to notify triggering Article 50 implicitly meant that Parliament authorised such triggering.
This case will therefore wind its way to the UK Supreme Court to decide whether the triggering of Article 50 met the requirements of the UK's constitutional setup. If it is decided that the triggering of Article 50 was not in keeping with the UK's constitutional requirements, then, under EU law, Article 50 was never validly triggered and the whole process would effectively be null and void ab initio (from the start).
Note that this case is purely about UK domestic law & constitutional arrangements and not about EU law per se.
Although I will be following it closely from personal interest in constitutional law & procedure, I would not hold my breath on it. The flexibility of the UK constitution will allow some manner to be developed for the trigger to be legalised retrospectively if need be.
On the other hand, the case filed by the Good Law Project asks whether, once triggered, Article 50 can be reversed/revoked under EU law and if so, how (unilaterally by the UK, by the UK and all EU27 or by the UK and a qualified majority of EU27). Presidents Tusk and Juncker have said that it can be, as has the author of Article 50 itself, but the law on this matter is unclear and can only be clarified by a reference to the CJEU, the highest authority on EU law, of which Article 50 is a part. A report of the European Parliament on the same question also was unable to come to a clear conclusion.
This case has been filed in the Scottish Court of Session after the case filed in the High Court of Ireland on the same matter was withdrawn, apparently because of the reluctance of the Irish government to participate.