Johnson sent the letter as required by legislation known as the Benn Act, but did not sign it and also sent a second letter - which he signed - that said a delay would be a mistake.

A hearing at the Court of Session in Edinburgh by Scotland's most senior judge Lord Carloway, and two other judges, was postponed until after Saturday October 20, the deadline for the extension letter to be sent.

The hearing is set to resume on Monday but Johnson's attempts to avoid requesting an extension raise questions about whether the court views the unsigned letter - and second contradictory letter - as obeying the law.

The legal challenge is on behalf of the Good Law Project, and founder Jolyon Maugham QC said the court's role was to uphold the law, not to act as parent to "a stroppy child".

"But the PM has made the request for an extension he promised he wouldn't make, the EU is considering that request, and the question whether the PM and Advocate General for Scotland are in contempt of court is one for the Inner House," he said.

If the three judges find Mr Johnson has failed to uphold the law, they could find him in contempt, with potential punishments including a fine or even a jail sentence.

Scottish National Party MP Joanna Cherry QC, who has been involved in bringing proceedings to court, described Johnson's actions as a "childish trick of not signing the letter and sending a contradictory covering letter".

"Our legal team are instructed to remind the court that as well as promising to comply with the letter of the Benn Act, the PM also promised not to seek to frustrate the purpose of the legislation," she said.

"It will be for the court to decide whether his actions in failing to sign the letter of request and sending a letter setting out his contrary intentions are in breach of the undertakings he gave them or a contempt of court."

Australian Associated Press