Boris Johnson could be jailed for contempt of court if he refuses to request an extension to the UK’s Brexit negotiations, according to a senior judge who is an uncle of Downing Street adviser Dominic Cummings.

Former appeal court judge Sir John Laws predicted the Supreme Court will hear a case against the prime minister within days if he fails to meet the 19 October deadline to seek an extension from Brussels.

The court could issue a writ known as an “order of mandamus”, which is used to command a government entity properly to carry out a non-discretionary function.

The rarely-used device is regarded as a drastic remedy and failure to comply with it would be regarded as contempt of court.

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The so-called Benn Act passed by a “rebel alliance” of opposition MPs and anti-no deal Tories earlier this month requires the PM to ask Brussels by 19 October to delay the date of Brexit to the end of January unless he has secured parliamentary approval for a deal or for a no-deal outcome.

Reports have suggested that Mr Cummings believes the courts would not be able to respond quickly enough if the deadline was missed to prevent the UK crashing out of the EU without a deal at the end of October.

But Sir John insisted the Supreme Court – which dramatically ruled on Tuesday that Mr Johnson’s suspension of parliamentary sittings was unlawful - could react swiftly to the legal challenge which would inevitably follow any failure by the PM to seek an extension.

He told the New Statesman magazine it was possible the court could convene within 24 hours, adding: “It can readily be within a week, I should have thought.”

Sir John said that the action taken by the court would depend on the discretion granted to Mr Johnson by the Act.

And he said: “If it is unambiguous, and plainly the unfulfilled duty of the prime minister, the court will … at least contemplate issuing an order of mandamus.”

If he still refused to go to Brussels to extend the Article 50 process, “ultimately he’d be guilty of contempt of court and sent to jail”, he said.