The decision by MPs to pass the Letwin amendment brings the legal action against the prime minister which resumes on Monday in Scotland’s highest civil court into sharp relief.
The court of session will resume its hearing into a request for an emergency interdict, or injunction, to force Johnson to send the Brexit extension letter required by the Benn act.
Johnson told the Commons on Saturday he was not legally required to do so – a position the court is now likely to test on Monday.
Ten days ago, Scotland’s most senior judge Lord Carloway, sitting with Lords Drummond-Young and Brodie, postponed a decision on enforcing the act’s provisions because they did not then know whether Johnson would have to uphold its provisions or would fail to do so.
That case, brought by Dale Vince, the millionaire owner of the Ecotricity green energy company, Jolyon Maugham QC, the anti-Brexit campaigner, and Joanna Cherry QC, the SNP MP will resume at noon on Monday.
If Johnson has failed to send the letter as required, by 11pm tonight, the court is expected to rule on Monday on whether to give Johnson 24 hours to do so or face being held in contempt of court.
Legal sources believe the prime minister is in significant legal peril. Lawyers for the UK government told the court on 9 October they knew the solemn pledges given at an earlier hearing that Johnson would comply with the act were legally-binding.
The UK government told the court of session on Friday 4 October the prime minister accepted “he is subject to the public law principle that he cannot frustrate its purpose or the purpose of its provisions. Thus he cannot act so as to prevent the letter requesting the specified extension in the act from being sent.”
And the court will also adjudicate on a second part of the application: an interdict forcing the UK government not to frustrate or undermine the intent of the letter, by attempting to sidestep the extension move.
If he or his ministers, or their proxies, try to subvert the request for an extension – say be sending a second letter asking the EU to ignore the extension application, they will also be at risk of contempt .
It is only if Johnson fails to send the letter and fails to adhere to the court’s interdict requiring him to do so that the court will consider the nuclear option. It has unique powers called nobile officium, which allow the court or its agent to send that letter to all 27 EU member states and institutions on Johnson’s behalf.
Those powers have never been used in such a way, and if the Scottish court uses them, would provoke another extraordinary constitutional confrontation. Assuming the court issues a definitive ruling next week, the case is expected to end up in the UK supreme court within days.