Boris Johnson’s two letters to the EU may be in contempt of parliament, Labour’s shadow chancellor John McDonnell has said.

The prime minister is facing court action on Monday after his letters to European Council president Donald Tusk were branded “pathetic” by Joanna Cherry, the MP who led a successful legal challenge against his unlawful suspension of parliament.

Johnson was forced by a humiliating defeat in the House of Commons on Saturday to submit a request to Brussels for an extension of the Brexit process beyond his 31 October deadline.

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But he insisted he would not negotiate an extension, and sent two letters to Mr Tusk – an unsigned message relaying parliament’s request for a Brexit extension and a signed letter from him setting out why he does not believe delay would be in the interests of the EU or UK.

Downing Street said it believes the move fulfils the requirements of the Benn Act, which required the prime minister to seek an extension if he was unable to secure parliamentary approval of his Brexit deal by the end of Saturday.

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But Scottish National Party MP Joanna Cherry dismissed the prime minister’s gambit as “pathetic” and said she would be pushing for legal action in Scotland’s highest court on Monday.

And Mr McDonnell told Sky News programme Sophy Ridge on Sunday: “It may well be in contempt of parliament or of the courts themselves, because they’re clearly trying to undermine the first letter.

“Not signing the letter is behaving a bit like a spoilt brat. Parliament made a decision and he should abide by it.”

Mr McDonnell, who addressed Saturday’s Final Say march in London, said Mr Johnson should not attempt to force a “meaningful vote” on his deal in the Commons on Monday, but should instead table his ratification legislation and allow MPs to “consider that bill properly”.

He indicated that Labour was unlikely to table a second referendum amendment during the passage of the bill, because tactically it was difficult for a motion from the leader of the opposition to gain cross-party support.

But he said a referendum motion would “inevitably” be put forward by backbenchers and suggested Labour would back it.

“We’ve always said that the people decide,” he said. “Boris Johnson should take the deal back to the people.”

Keir Starmer, the shadow Brexit secretary, branded Mr Johnson’s actions “childlike”.

Sir Keir told BBC1’s The Andrew Marr Show: “The law is very clear. He should have signed one letter in accordance with the law. He’s being childlike. If we crash out because of what he’s done with the letters in 11 days’ time without a deal he bears personal responsibility for that.

“I’m sure there will be court proceedings.”

Sir Keir confirmed that Labour will “fight amendments” to Mr Johnson’s Withdrawal Agreement Bill as it passes through parliament to ensure a close economic relationship with the EU over the long term.

“This is a very, very bad deal and it has within it a trapdoor to no deal at the end of next year,” he warned.

Dominic Raab, the foreign secretary, insisted that Mr Johnson’s letters met the requirements of the Benn Act.

“The prime minister has done exactly what he said he was going to do. He has complied with the law,” Mr Raab told Marr.