The UK government has promised a court that Boris Johnson will send a letter to the EU seeking an extension to article 50 as required by the Benn act.

The undertaking appears to contradict the prime minister’s statements on the UK leaving the EU on 31 October regardless and unattributed claims from Downing Street that he will find a way to sidestep the act.

The pledge has been given in legal papers submitted to the court of session in Edinburgh after anti-Brexit campaigners began legal action to force Johnson to uphold the act’s requirements.

The European Union (Withdrawal) (No. 2) Act states that if Westminster does not agree to a Brexit deal by 19 October, the prime minister has to write to the EU seeking an extension to article 50 until 31 January.

The UK government has refused to release copies of its submissions in this case to the media despite repeated requests by the Guardian, the BBC and other news organisation.

Key excerpts of its pledge were read out instead by Aidan O’Neill QC, the lawyer for the green energy millionaire Dale Vince, the SNP MP Joanna Cherry QC, and the lawyer and anti-Brexit campaigner Jolyon Maugham QC.

Maugham then tweeted extracts of the document.

Here are the paragraphs Aidan O'Neill QC, Counsel for @DaleVince, @joannaccherry and myself, has just read out. They are from the written case of the Prime Minister and if the Prime Minister promises the court he will comply we need take no further time. Why will he not? pic.twitter.com/YnYLJ0iMhn — Jo Maugham QC (@JolyonMaugham) October 4, 2019

It states the prime minister accepts “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.”

O’Neill told Lord Pentland, the judge hearing the case, that Johnson had repeatedly contradicted that position, including in the Commons on Wednesday, by insisting the UK would leave on 31 October come what may.

As a result, O’Neill said, the court still needed to issue legally binding orders to force Johnson to comply with the Benn act in an interdict, or injunction. If the prime minister refused to do so, O’Neill could return to court to ask for Johnson to be fined or jailed, he added. No 10 declined to comment.

Johnson has insisted repeatedly that he will “get Brexit done” by 31 October.

​In his party conference speech on Wednesday, he said: “What leavers want​, ​what remainers wan​t, what the whole world wants is to be calmly and sensibly done with the subject, and to move on​: ​and that is why we are coming out of the EU on 31 October, come what may​”.

As he laid out his Brexit proposals to MPs on Thursday, he told them: “We will be leaving on 31 October, deal or no deal.”

However, some cabinet ministers in Manchester repeatedly hinted that Downing Street believes it can circumvent the Benn act in some way, so that Brexit would still go ahead on deadline.

Steve Baker, chair of the European Research Group, appeared to suggest on Friday that he had been reassured the government still intended to leave on 31 October, come what may. “All this means is that government will obey the law,” he wrote.

A source confirms all this means is that Government will obey the law.



It does not mean we will extend.



It does not mean we will stay in the EU beyond Oct 31.



We will leave. https://t.co/LuVt45rMAr — Steve Baker MP (@SteveBakerHW) October 4, 2019

The UK government’s lawyer, Andrew Webster QC, argued that no order by the courts was necessary because the prime minister had already issued a clear undertaking to the court he would obey the Benn act’s provisions.

Claiming it would undermine the government’s negotiations with the EU to have its hands bound by the courts, Webster said: “What we have here is a clear statement made on behalf of the prime minister and on behalf of the government as to what he will do in respect of the requirements of the 2019 act. They have been put on record so they that there can be no doubt. [The] prime minister will comply with the law.”

But O’Neill quoted statements from a Downing Street source in a BBC story which O’Neill said directly contradicted Webster’s promises and proved why an interdict – an injunction to prevent to prevent Johnson breaching the Benn act – was needed.

The BBC source said the Benn act’s “very specific narrow duty” did not prevent the government from “other communications, private and public” which, O’Neill said, would thwart the act’s duties to seek an extension to article 50 after 19 October.

The BBC source said “the government is making its true position on delay known privately in Europe and this will become public soon.” O’Neill told the court this was proof of the government’s “spin, misunderstanding and misinterpretation” of the act. It was proof, he said, it could not be trusted to honour what the court had been told.

Lord Pentland, the judge, said it was entirely legitimate for people to question or distrust promises from public bodies. “The negotiations are one thing. Compliance with the law is another,” he told Webster. “Public authorities often give undertakings to satisfy a whole host of challenges.”

Pentland is due to give his ruling on the interdict application on Monday, before a parallel case is heard on Tuesday where the same group of anti-Brexit campaigners will ask other judges to use powers unique to the Scottish courts to send the Benn act letter on Johnson’s behalf if he refuses to do so.