Scotland’s highest civil court is to hear arguments over whether the Prime Minister can be forced to extend Article 50.
The legal action – led by businessman Vince Dale, SNP MP Joanna Cherry QC and Jolyon Maugham QC – will ask the Court of Session to require Boris Johnson to seek an extension to avoid leaving the EU without a deal.
It could also later see a court official being given the power to sign the letter if the Prime Minister refuses to do so.
The so-called Benn Act was passed by Parliament last month requiring the Government to ask for an extension until January 31 if an agreement is not reached with the EU by October 19.
Opponents to no-deal are suspicious the Prime Minister will try to thwart the legislation in order to fulfil his vow to leave the EU on October 31 “do or die”.
Ms Cherry said: “Like much of what Boris Johnson says, there is a gulf of truth between the obvious facts of the matter and what he and his Government have been saying.
“He cannot be trusted, and this court action is about ensuring he abides by the law.
“If Boris Johnson tries to defy the law and defy both the Holyrood and Westminster parliaments by crashing out of the EU without a deal – then we are calling on the Scottish courts to uphold the law.”
Speaking to Good Morning Scotland on Friday, Ms Cherry said: “We’re not bringing this case because we think there’s any loophole in the Benn Act.
“We’re bringing this case because we’re dealing with a British Prime Minister who brags about not obeying the law and has form for doing things that are unlawful.
“Boris Johnson is not above the law, whether in Scotland or south of the border, thanks to the decision of the UK Supreme Court.”
The SNP MP also said she hopes the Prime Minister will be clear on his position about obeying the Benn Act.
She said: “If this court case achieves nothing else but getting him to be clear about what his position is, then it will have achieved something.”
Ms Cherry previously won a Supreme Court case against the Government which found the planned prorogation of Parliament until October 14 was unlawful.
The Outer House court hearing on Friday will seek an order to ensure Mr Johnson requests an extension to the Article 50 process if he refuses to abide by the terms of the Benn Act.
It will go to the Inner House on Tuesday, where campaigners will ask the Scottish judges to use the unique power of “nobile officium” to empower a court official to sign the extension letter if the Prime Minister refuses to do so.