A supreme court judge has raised the prospect that Theresa May would have to comprehensively replace existing EU legislation before the government could even begin Brexit, in a move that could seriously delay the process.

In a speech that angered leave campaigners, Lady Hale said the supreme court judges could go further than simply forcing May to publish a short piece of legislation to approve the triggering of article 50.

The deputy president of the court said that next month’s case – in which the supreme court will hear the government’s appeal against a high court ruling that MPs must approve the triggering of article 50 – raised “difficult and delicate issues” about the relationship between government and parliament.

“Another question is whether it would be enough for a simple act of parliament to authorise the government to give notice, or whether it would have to be a comprehensive replacement of the 1972 act,” she said in comments to law students in Kuala Lumpur that were published online on Tuesday. The European Communities Act 1972 took the UK into the then European Economic Community.



Hale set out the arguments on both sides of what is expected to be the most constitutionally significant case ever heard by the supreme court. She told the students that while the referendum had produced a majority of 51.9% in favour of leaving the EU, “that referendum was not legally binding on parliament”.

She put forward the argument that the government was likely to make, saying it would suggest: “The basis on which the referendum was undertaken was that the government would give effect to the result. Beginning the process would not change the law.”

The comments come amid reports that the government has prepared a short three-line bill aimed at helping May stick to her March deadline for triggering article 50 if the supreme court ruled that was necessary. Ministers are said to have drawn up short legislation that would be difficult to amend.

Hale is one of 11 judges due to hear the case at the supreme court, which comes after the high court ruling immediately triggered a backlash from some politicians and rightwing media. The Daily Mail described the judges as “enemies of the people”.

The former work and pensions secretary Iain Duncan Smith said Hale had pro-EU views and warned that it was not the job of judges to tell parliament what to do. Duncan Smith said: “This is a very big step. If they were to do this it’s a constitutional crisis. What the judges will decide on at the supreme court is whether or not the government can use its executive powers to trigger article 50.



“It is not their job to tell parliament … how they should go about that business, that’s for parliament to decide.”

He said “the individual concerned” had always opposed Britain leaving the EU, and said that he did not believe that would be a majority view in the supreme court.

The Conservative MP Dominic Raab said: “If judges dip their toes in political waters by making speeches outside the courtroom, they are asking to get splashed back.”

Raab did not question whether the comments suggested which way Hale would sway but argued that she simply should not have made any public comments. “I’m all for democratic debate. But you can’t have it both ways. If such a senior judge muses in public about a pending supreme court judgment, the judiciary can hardly scream blue murder if politicians, the media or public respond,” he said.

After the outcry over the high court ruling, the prime minister was told to calm the “mob”, with the former attorney general Dominic Grieve saying the coverage “started to make one think that one was living in Robert Mugabe’s Zimbabwe”.

Other Brexit supporters also lashed out at Hale’s lecture. Sir Bill Cash, a Tory MP and chairman of the European scrutiny committee, said: “I am astounded that a justice of the supreme court would venture into this territory before hearing the evidence of the case. It appears completely contrary to the proper relationship between the courts and parliament for her to suggest what parliament should do.”

A supreme court spokesman said that Hale was simply presenting the arguments from both sides of the article 50 appeal in an impartial way for an audience of law students as part of a wider lecture on constitutional law, and it was proper for judges to set out arguments in high-profile cases to help public understanding of the issues involved in an even-handed way.

“One of the questions raised in these proceedings is what form of legislation would be necessary for parliament to be able to lawfully trigger article 50, if the government loses its appeal,” the spokesman said. “A number of politicians have raised the same question. Though it was not dealt with explicitly in the high court judgment, it is not a new issue. In no way was Lady Hale offering a view on what the likely outcome might be.”

• This article was amended on 18 November 2016. An earlier version said incorrectly that 51.9% of the British electorate had voted to leave the EU.