Scotland’s highest court has delayed a decision on whether the prime minister has fully complied with a law requiring him to ask for a Brexit extension.

Boris Johnson sent an unsigned letter to Brussels asking for a delay, along with a signed letter saying he believed that doing so would be a mistake.

Campaigners want the judges to enforce the so-called Benn Act, which is aimed at preventing a no-deal exit.

The UK government argued that it had fulfilled its legal obligations.

But Lord Carloway said the case should be continued until those obligations had been complied with in full.

A date for the next hearing at the Court of Session has yet to be set.

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The original case was brought by SNP MP Joanna Cherry, businessman Dale Vince and QC Jolyon Maugham.

They said they had asked for a further extension on Monday in an attempt to maintain the pressure on Mr Johnson.

Mr Maugham said he was “delighted” with the court’s decision.

“It is a pity to have to say it, but this is not a prime minister who can be trusted to comply with the law. And because he cannot be trusted he must be supervised,” he said.

The court was originally asked earlier this month to consider using “nobile officium” powers to request a Brexit extension on the prime minister’s behalf – but the judges delayed making a ruling until the political situation become clearer.

Ms Cherry said the legal action had already been instrumental in forcing Mr Johnson to send the request for an extension late on Saturday.

She told the BBC’s Good Morning Scotland programme: “After all his huffing and puffing, the prime minister has had to climb down and seek an extension.

“And I think he was trying to spin that by not signing the letter and issuing another letter.

“The good news is that the EU have ignored that nonsense and are taking the request seriously.

“It will be for the court to decide whether or not the prime minister has broken his promise to the court. His promise wasn’t to me or any of the other petitioners – it was to the court.”

Why is this back in court again now?

The Benn Act, passed in September, required Mr Johnson to request a three-month Brexit delay unless he could pass a deal or get MPs to approve a no-deal exit by 19 October.

Fearing he might find a way to circumvent this, campaigners sought to provide a “safety net” by asking Scotland’s highest court to use “nobile officium” powers to write a letter on the prime minister’s behalf if he failed to do so.

An earlier hearing was told Mr Johnson had given an undertaking to “fully comply” with the law and that he accepted he could not “frustrate” the purpose of the act.

The judges decided that the political debate had still to “play out” and therefore delayed making a decision.

They agreed the court should sit again on 21 October by which time they hoped the circumstances would be “significantly clearer”.

At a special sitting of the House of Commons on Saturday, MPs passed passed an amendment, put forward by Sir Oliver Letwin, delaying approval of Boris Johnson’s Brexit deal. This meant, by the terms of the Benn Act, he had to write to the EU requesting an extension.

He did send this request, along with the second letter, saying he believed a further Brexit delay was a mistake, late on Saturday.

What is the nobile officium?

The procedure of petitioning the nobile officium is unique to Scots law. Its name is a Latin term meaning the “noble office”.

The procedure offers the opportunity to provide a remedy in a legal dispute where none exists.

In other words, it can plug any gap in the law or offer mitigation if the law, when applied, would be seen to be too strict.

In this case, it could have seen an official of the court sign a letter to the EU requesting a Brexit extension, as set out in the Benn Act, should the prime minister have failed to do so.