Pro-remain campaigners are preparing to launch contempt proceedings against Boris Johnson if he attempts to renege on promises to seek an extension to Brexit.

Jolyon Maugham QC has sought legal advice on starting court proceedings for contempt next week in the Scottish courts, after senior judges in Edinburgh delayed a decision on ordering the prime minister to comply with the Benn act.

Lord Carloway, Scotland’s most senior judge, has already cleared time for an emergency hearing in the court of session at noon on Monday 21 October, where he could issue court orders forcing Johnson to send a letter to the EU asking for an extension to article 50 until 31 January.

The UK government’s lawyers promised the court Johnson would honour the requirements of the European Union (withdrawal) (No 2) act by sending the letter if he fails to secure support in the House of Commons for Brexit by Saturday 19 October.

The government also promised neither the prime minister nor any other minister or official would try to frustrate the Benn act by sidestepping it or undermining its provisions, despite Johnson’s repeated pledge the UK would leave the EU on 31 October come what may.

But Maugham, who took the court action against Johnson along with Dale Vince, the millionaire owner of the Ecotricity green energy company, and Joanna Cherry QC, a Scottish National party MP, said he believed the prime minister was still trying to do this.

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He confirmed he had asked a leading Scottish QC to prepare a civil action for contempt against Johnson, which would be lodged with the courts next week if there were fresh evidence the prime minister planned to obstruct moves towards an extension of article 50.

“It would be a terrible mistake for the prime minister to imagine such a gross contempt would go unpunished,” Maugham said after the ruling on Wednesday.

“This is serious. This isn’t some trivial administrative breach. This is a deliberate defiance of the will of parliament and the will of the court. If he is in breach of the law, there will be very serious consequences for him personally.”

Maugham, Vince and Cherry included a proposal to the court last week that Johnson could be jailed or fined for breaching the act. That was not formally proposed in the court hearing last Friday before Lord Pentland, but remains part of their legal submissions.

It became clear on Wednesday that Johnson’s hopes of securing a last-minute deal with the EU were doomed after the collapse of talks with the German government and disclosure from the Irish government that there was no prospect of a deal before 31 October.

Johnson has asked for a rare Saturday sitting of the Commons on 19 October for emergency debates and votes.

In a highly unusual but significant ruling, Carloway, sitting with Lord Drummond Young and Lord Brodie, said the court needed to wait to see what happened next week before deciding whether it had to enforce the Benn act.

“The political debate requires to be played out in the appropriate forum,” the judges ruled. “The court may only interfere in that debate if there is demonstrable unlawfulness which it requires to address and to correct. At present, there has been no such unlawfulness.”

The three judges agreed with Pentland, who ruled on Monday that there was no proof Johnson was actively taking steps to undermine the Benn act and the government was entitled to have political arguments about its Brexit strategies.

Carloway said the situation was very fluid and if Johnson did try to thwart or sidestep the act, this would become clearer after midnight on 19 October. If he breaks his promises to the court, it could issue an interdict, or injunction, ordering Johnson to send the extension request, or it could do so, using unique powers open to Scottish judges known as nobile officium (noble office).

That empowers a court to take action on a legal matter where the responsible person fails to do so, or in order to remedy a deficiency. It would be unprecedented but in that situation, the court would write, sign and send the letter specified in the legislation to Brussels and the other 27 EU member states.

The judges also hinted strongly in their ruling that they could find Johnson in contempt of court, without a specific application by Dale, Maugham and Cherry, if the prime minister failed to honour the promises government lawyers had made to the court.

The government had refused to give the court sworn undertakings, a legally binding pledge that Johnson would do as he had promised, but Carloway implied the “detailed and specific averments” made by its lawyers were enough. Therefore, coercive orders, such as interdicts or nobile officium rulings, were not needed at this stage.

Cherry said Johnson and his aides had to understand the significance of the court suspending judgment. It “surely showed there is some doubt lodged in their minds that the promises made to the court will be adhered to”, she said.