Campaigners have launched a fresh legal challenge to force the UK Government to ask the European Union for an extension to even if Boris Johnson refuses.
The Prime Minister has previously said that he would rather be "dead in a ditch" than ask the EU for a delay to Brexit beyond the October 31 deadline.
There have been fears that Johnson may refuse to ask for the extension despite a law being passed by Parliament to avoid a no-deal Brexit.
Now, a legal challenge launched at the Court of Session in Edinburgh would see the court step in if the Prime Minister fails to comply with that law.
If the challenge is successful, it would give the court the power to send the letter to the EU in the PM's absence.
Johnson is compelled to ask for an extension to the end of January if he fails to convince MPs to back a Withdrawal Agreement or support a no-deal by October 19, after Parliament passed the so-called Benn Act.
SNP MP Joanna Cherry QC and barrister Jolyon Maugham QC said they issued proceedings at Scotland's highest court, the Inner House of the Court of Session, on Thursday.
Maugham said that they have selected Scotland for the case because judges there have the "nobile officium" power that could allow the court to sign the letter if the PM refuses.
"The rule of law is not a thing to be grifted - not even by the Prime Minister," he said.
"We expect that the Inner House will be mindful of the deadline set out in the Benn Act, and will deal with the matter speedily."
The campaigners won a case that sent political shockwaves through the UK and prompted calls for the Government to bring back Parliament, when three senior judges ruled that the PM had acted unlawfully.
Johnson went on to deny lying to the Queen to secure the five-week prorogation of Parliament.
He insisted he sought the suspension for the Government to set out a new legislative agenda in a Queen's Speech when MPs return on October 14.
The judges concluded on Wednesday that the Prime Minister had acted with the "purpose of stymying Parliament".
But the Government is appealing and the case is scheduled to go to the Supreme Court on Tuesday.
The same court will also hear an appeal in a case brought by campaigner Gina Miller over the suspension.
The High Court in London rejected her case, saying the decision to prorogue is "purely political" and cannot be capable of challenge in the courts.
