Lawyers representing SNP justice spokeswoman Joanna Cherry, legal expert Jo Maugham and businessman Dale Vince, argued the Prime Minister could not be trusted to stick to the terms of legislation passed by opposition MPs last month.
The trio want Scotland’s highest civil court to use its power of nobile officium to send a letter requesting an extension if no deal has been reached with the EU by October 19.
The rarely-used power allows judges to take action to ensure a legal requirement is upheld if the official responsible fails to do so.
Aidan O’Neill QC, representing the petitioners, said Mr Johnson and his political allies had repeatedly indicated they would try to get around the terms of the Benn Act to ensure the UK left the EU by a deadline of October 31.
He said: “This is politics being used to subvert the rule of law. Saying one thing in court and another in public.”
He continued: “One should not be taken in by any of kind schtick that the prime minister is simply an overgrown schoolboy playing at being prime minister, as if he was Just William leading his cabinet band of outlaws.
“This is serious stuff. This is calculated. This is deliberately focus-grouped, workshopped and war-gamed to appeal to their base.”
But Andrew Webster QC, representing the UK Government, said assurances already given to the court carried “significant weight” which could be relied upon.
It was revealed in court papers made public last week that Mr Johnson accepted the terms of the Benn Act. Mr Webster added that “no weight” should be given to unattributed statements made to several newspapers in recent days about the prime minister’s intentions.
Mr O’Neill told the court that sending a letter on behalf of the prime minister was unprecedented, “but these are unprecedented times”.
Lord Carloway, sitting with Lord Drummond Young and Lord Brodie in the Inner House of the court, said a decision on the nobile officium would be made at 11am on Wednesday.
One option open to the court is to continue the case until after the October 19 deadline to ensure the Benn Act is observed by the UK Government.
The judges will also rule at the same time on an appeal lodged against Lord Pentland’s ruling on Monday, which dismissed a petition aimed at forcing the Prime Minister to request a Brexit extension if no withdrawal deal is secured with the EU by October 19.
Lord Pentland ruled it was not necessary to compel Mr Johnson to comply with the terms of the so-called Benn Act given the “unequivocal assurances” of Boris Johnson and the Government made before the court.
In a statement issued after today’s proceedings, Mr Maugham said: “As it seems to me, the court is plainly, and understandably, concerned to avoid, if at all possible, a destructive clash between the Executive and the rule of law.
“The Government has clarified that it will send the Benn Act letter and it will not frustrate the intentions of the Benn Act. And the Government has clarified there may be recourse to the court if not.
“The court, I think, is likely to require those clarifications as sufficient, at the moment.
“These are difficult times and, whatever the outcome, I am grateful to both the Outer and the Inner House for giving the matter its careful consideration.”