Shadow chancellor John McDonnell has insisted Boris Johnson may be in contempt of Parliament over Brexit.
Mr McDonnell hit out at the way the Prime Minister distanced himself from a legally required request to the EU for a Brexit extension which he refused to sign.
The shadow chancellor accused Mr Johnson of “behaving a bit like a spoilt brat” over the issue.
Mr Johnson stressed to Brussels he was only sending the communication at Parliament’s bidding.
After suffering an embarrassing defeat in the Commons over his Brexit plans, the Prime Minister got a senior diplomat to send Brussels an unsigned photocopy of the call by MPs to delay withdrawal from the bloc, with a cover note stressing his detachment from the move.
In a second note to European Council president Donald Tusk, the PM said a Brexit extension would be “deeply corrosive”.
Speaking on Sky News’s Sophy Ridge on Sunday, Mr McDonnell said: “He may well be in contempt of Parliament or the courts themselves because he’s clearly trying to undermine the first letter and not signing the letter.
“He’s behaving a bit like a spoilt brat.
“Parliament made a decision, he should abide by it and this idea that you send another letter contradicting the first, I think it flies in the face of what both Parliament and the courts have decided.”
The PM’s move came after Mr Johnson told the Commons he will not negotiate a fresh Brexit extension with the EU.
Mr Tusk tweeted: “The extension request has just arrived. I will now start consulting EU leaders on how to react. #Brexit”
The move followed a special Saturday sitting of the Commons where MPs voted by 322 to 306 in favour of an amendment withholding approval of his Brexit deal until legislation to implement it is in place.
The amendment tabled by former Cabinet minister Sir Oliver Letwin was intended to force him to comply with the so-called Benn Act requiring him to seek a Brexit extension.
The PM rang European leaders, including Mr Tusk, on Saturday declaring that the letter “is Parliament’s letter, not my letter”.
The Benn Act set a deadline of 11pm on Saturday for the Prime Minister to get a deal if the UK is to leave on October 31, otherwise he is supposed to seek an extension.
Brexit Party leader Nigel Farage said he would rather have a withdrawal extension followed by a general election than see Boris Johnson’s latest deal being passed in the Commons.
He told Sky News: “I want a general election, so an extension for a few weeks into which we can have a general election is a much better outcome than signing up to a treaty that becomes part of international law that binds us in foreign policy and in many, many other areas.”
Commons Leader Jacob Rees-Mogg said the Government was planning to give MPs a fresh chance to have a “meaningful vote” on the deal hammered out by Mr Johnson with Brussels on Monday.
Speaker John Bercow said he would rule on Monday whether it was in order for the Government to hold the vote, after not pressing ahead with it on Saturday following its defeat on the Letwin amendment.