Labour have declared "spoilt brat" Boris Johnson may be in contempt of court after he sent two letters to Brussels over his Brexit defeat.
The Prime Minister last night sent a a legally-required request to the EU for a Brexit extension, but refused to sign it in a political stunt - saying he was only sending it at Parliament's bidding.
And in a second note to European Council President Donald Tusk, the PM said a Brexit extension would be “deeply corrosive”.
Shadow Chancellor John McDonnell told Sky News: “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.”
Both Mr McDonnell and Shadow Brexit Secretary Sir Keir Starmer said the Prime Minister's behaviour could end up being challenged in court.
Sir Keir - the former Director of Public Prosecutions - told the BBC's Andrew Marr Show: "I’m sure there will be court proceedings.
"But the first thing we’re going to do in Parliament is deal with that issue.
"The second thing is then if he brings in a Bill to implement his deal we will fight amendments on that because the deal is a very very bad deal and it has within it a trapdoor to no deal at the end of next year."
Branding the PM "childlike", Sir Keir said: "The law is very clear he should have signed one letter in accordance with the law.
"He’s being childlike and if I don't think it’ll happen but 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.
"That’s the first fight."
Asked if the behaviour could be challenged in court, Mr McDonnell added: "It may well be. Others will decide that matter.
"But a number of lawyers have already commented that this is seeking to undermine a decision of Parliament and therefore flies in the face of both Parliament and court decisions.”
Even Brexit Party leader Nigel Farage said it was “not very prime ministerial”.
A senior No10 source told the Sunday Telegraph Remain campaigners "will say this is illegal [but] we don't think it is."
But a former Tory Cabinet minister told the Observer it was "against the spirit of the Benn Act”, the law which forced him to ask for a delay.
The ex-minister said: “I think this will end up in the courts again.
"It will also put government law officers in a very uncomfortable position."
Two weeks ago, Scotland's Court of Session ruled against campaigners who wanted an order to stop Boris Johnson "frustrating" the delay request with a second letter.
The UK government won over the court by finally promising, in writing, that it would obey the Benn Act after all. Campaigners could now argue that, in effect, Mr Johnson did not obey the law.
One prominent Remainer told the Mirror Scotland's Court of Session is due to sit again tomorrow, but there is a difference of opinion.
"There appear to be two genuine sets of views about the second letter," they said.
"One is the second letter is unlawful, the other is that it’s legally irrelevant.
"Either way everyone agrees the legally significant one is the first one and the EU will just ignore the other."
They added: "The fact the Benn letter is unsigned is a total red herring and childish behaviour by Johnson."
Meanwhile, Michael Gove said the Government had triggered the next stage of Operation Yellowhammer no-deal plans because the Letwin amendment had increased the chances of leaving without a pact.
No10 hopes to stage another Meaningful Vote tomorrow on his Withdrawal Agreement, if Commons Speaker John Bercow gives the go-ahead.
But Mr McDonnell said another vote would be “pointless” and a “political stunt”.
The PM received a boost when former Tory Cabinet Minister Amber Rudd revealed she would back his deal.
Downing Street is expected to introduce the Withdrawal Agreement Bill to Parliament today, with debates beginning on Tuesday.
Shadow Brexit Secretary Sir Keir Starmer indicated Labour would back an amendment for a second referendum.
