Prime Minister Boris Johnson has sent two letters to European Council president Donald Tusk – an unsigned message relaying parliament’s request for an extension to Brexit and a 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 beyond his 31 October deadline if he was unable to secure parliamentary approval of his Brexit deal by the end of Saturday.

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

Responding to the news of the two letters, Ms Cherry – who led a successful legal action against Mr Johnson’s five-week suspension of parliament – said: ”This is pathetic. Boris Johnson promised the Scottish court he would comply with the Benn Act and not seek to frustrate it.

“Looks like he’s breaking both promises. Fortunately, no need to raise new proceedings – our existing case is back in court on Monday.”

In his signed letter, Mr Johnson wrote that the request for a Brexit extension would be submitted not by him, but by the UK’s permanent representative in Brussels Sir Tim Barrow.

He said he remained confident that he can complete the process of ratifying the Brexit deal reached with the other 27 EU states last Thursday by the end of the month.

And he said: “While it is open to the European Council to accede to the request mandated by parliament or to offer an alternative extension period, I have made clear since becoming prime minister - and made clear to parliament again today - my view, and the government’s position, that a further extension would damage the interests of the UK and our EU partners and the relationship between us.

“We must bring this process to a conclusion so that we can move to the next phase and build our new relationship.”

In an apparent hint to Brussels to take its time over responding, Mr Johnson said he recognised that a new summit of EU leaders may be needed to consider the extension response, and said that he would be happy to attend to answer questions.