Verdict on Thursday will determine whether or not the UK’s decision to leave European Union must be subject to a vote by parliament

The lord chief justice is to deliver the high court’s momentous decision on whether parliament or the government has the constitutional power to trigger Brexit.

After less than three weeks considering the politically charged case with two other senior judges, Lord Thomas of Cwmgiedd will read out a summary of their decision at 10am on Thursday to a packed courtroom in London’s Royal Courts of Justice.

In order to prevent leaks of the market-sensitive ruling, which involves a large number of parties, preliminary drafts of the judgment have unusually not been sent out in advance to the lawyers.

The outcome of the case, which ventures into constitutionally untested ground, will resolve whether MPs or ministers have the authority to formally inform Brussels about whether the UK intends to leave the European Union.

What could happen if the article 50 legal challenge is successful? Read more

The legal dispute focuses on article 50 of the treaty on European Union, which states that any member state may leave “in accordance with its own constitutional requirements” – an undefined term that has allowed both sides to pursue rival interpretations.



The arguments deployed during the three-day hearing last month appear, at the very least, to have reinforced political pressure for parliament to be given a greater role in negotiating Brexit.

Whether the high court finds in favour of the claimants or Theresa May’s assertion that the prime minister has power under the royal prerogative to inform Brussels of the UK’s intention to leave, one side or the other is likely to appeal to the supreme court.

However, there has been speculation that the government could decide not to appeal if it loses, calculating that enough MPs will feel bound by the result of the referendum to vote to leave the EU. There may be stiffer opposition in the House of Lords.



A case heard in the high court would normally go up to the court of appeal, but arrangements have been made for the case to “leapfrog” directly to the supreme court if permission is granted.



Space has been cleared in the supreme court’s diary for a possible hearing on 7 and 8 December. It would be heard by at least nine justices; an odd number is required to prevent a tie.



Because the initial case was heard in the high court, it has not been televised. The three judges who considered the challenge were the lord chief justice, Lord Thomas; the master of the rolls, Sir Terence Etherton; and Lord Justice Sales.

Facebook Twitter Pinterest Gina Miller. Photograph: Mark Thomas/Rex/Shutterstock

The two lead claimants are Gina Miller, a businesswoman and philanthropist, and Deir dos Santos, a hairdresser. Both are British nationals.

They have been supported by other interested parties, including the crowd-funded People’s Challenge, whose members live in England, France, Gibraltar, Northern Ireland, Scotland and Wales. It has to date raised more than £100,000 from almost 5,000 supporters to finance the case. Another group, Fair Deal for Expats, is backed by British expatriates living abroad in 10 EU states.

A related challenge heard in Northern Ireland’s courts – emphasising the complexities of devolution legislation – has already been won by the government. That challenge may also be appealed to the supreme court and joined with the London claims.



Lawyers for the claimants were relatively upbeat about their prospects at the end of the hearing in mid-October. The challenge has been described as one of the most important constitutional cases in generations.

At the the Conservative party conference, May said she intended to trigger article 50 by the end of March 2017.

Opening the case for the government, the attorney general, Jeremy Wright QC, said that the claimants were attempting to invalidate the referendum result.



Facebook Twitter Pinterest Jeremy Wright QC. Photograph: Hannah Mckay/EPA

In their final submissions, government lawyers revealed that parliament is “very likely” to be asked to ratify any future treaty agreement with the European Union.

The lack of agreement over fundamental political principles has prompted concerns that the UK’s unwritten constitutional arrangements may need to be updated.

In a speech released on Wednesday, the president of the supreme court, Lord Neuberger, suggested that “there is undoubtedly a case for saying that the time has come for the United Kingdom to adopt a formal written coherent constitution.



“However, in that context, the typically British and pragmatic argument ‘If it ain’t broke, don’t fix it’, has obvious resonance. Having said that, there are some people who feel that it is broke, and others who feel that there is a duty to act before it gets broke.



“But, even those people must accept not merely that the grass always seems greener on the other side of the fence, but also that experience shows that the fact that a particular arrangement works well in one country, even in most countries, does not necessarily mean that it will work here.”