The prime minister, having promised that he would never make such a request, was forced to do so after MPs voted to withhold approval for his withdrawal agreement.
Labour shadow chancellor John McDonnell said Mr Johnson “has to abide by the law” and may be in contempt of parliament, adding: ”He’s behaving a bit like a spoilt brat.”
However Michael Gove, the government minister in charge of Brexit preparations, insisted that the UK would still leave the EU on 31 October.
“We are going to leave by October 31,” he told Sky News. ”We have the means and the ability to do so and yesterday... we had some people who voted for delay, voted explicitly to try to frustrate this process and to drag it out.”
The government is planning to hold a “meaningful vote” on Mr Johnson’s deal on Monday but opposition MPs are seeking amendments to protect against a no-deal Brexit and hold a second referendum.
Supporters of a Final Say referendum are being urged to sign a letter which calls on officials in the UK and the EU to do everything they can to give the people a vote on the final Brexit deal.
Follow events as they happened in our liveblog below:
Good morning and welcome to The Independent's live coverage of the latest developments in the Brexit saga as we approach the 31 October "deadline".
It's the morning after Super Saturday and Boris Johnson has sent not one but two letters to the EU following his defeat in the House of Commons on the Letwin amendment.
One was an unsigned message relaying parliament’s request for an extension to Brexit and the other was 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.
Shadow chancellor John McDonnell accused the Prime Minister of "behaving a bit like a spoilt brat" over Brexit.
Speaking on Sky News's Sophy Ridge on Sunday he 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."
Asked if Labour could back the Brexit deal, Mr McDonnell said: "When the Bill comes forward, what we'll try to do, as in the normal way, put amendments to that Bill and see whether or not we can ensure that it meets the criteria that we've set out.
"The problem that we've got is this has become quite a fundamental choice about the future of our economy. Do we want to go down the Boris Johnson proposals of diverging from our major trading partner and deregulating our economy, undermining workers' rights, consumer and environment rights? No we don't.
"So what we'll try and do is, of course, try and amend that legislation and see if we can get agreement in Parliament."
Lawyers and legal commentators have been debating whether or not Boris Johnson's two letters ploy is in breach of the Benn Act - which compelled the prime minister to request a delay to Brexit beyond 31 October.
Here David Allen Green suggests there is no need to go to court. Donald Tusk, president of the European Council, has confirmed receipt of the request and it is now up to the EU member states to decide whether to grant an extension or not.
However Lib Dem Brexit spokesperson Tom Brake MP suggests the prime minister will be hauled back to court on Monday.
Brexit Party leader Nigel Farage says he is in favour of a short delay to Brexit to hold a general election, describing Boris Johnson's deal as "rotten".
"I want to leave on the 31 of October, but I'll warn everybody that if this treaty goes through nothing will have changed at all, and I think far better to have a short delay and a general election where we might solve this," 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.
"We are going to have to be on a level playing field with the rest of Europe which means we still haven't taken back control of our laws - this is not Brexit.
"Its a new EU treaty, it binds us. All it does is take us on to the next stage of negotiations."
Former home secretary Amber Rudd, who voted for the Oliver Letwin amendment forcing the prime minister to seek an extension, has said she will support Mr Johnson's deal in the Commons.
Ms Rudd admits it is "not as good as Theresa May's deal" and will damage the UK's economy, reducing the GDP by between four and six per cent.
She describes it as a "compromise" - because it's better than no-deal.
"We have to make sure that we don't leave with no deal," Ms Rudd tells Sky News. "I want to support it and I will and I think, not all of us, but most of us former Conservatives, who supported the Letwin amendment will do so as well.
"I think it's absolutely right to say we don't want to leave with no deal but we do want to leave with a deal and this deal from the Prime Minister is good enough for me."
Yesterday also saw an estimated one million people march on parliament calling for a Final Say referendum on any Brexit deal.
Now supporters of a public vote are being urged to sign a letter which will be delivered simultaneously to Downing Street and the Houses of Parliament, as well as the European parliament and the European Council, before 31 October.
More than 130,000 people have already signed in just over 24 hours.
Sir Keir Starmer, the shadow Brexit secretary, has also piled in on Boris Johnson, accusing him of being "childlike".
He told the BBC's Andrew Marr show: "The law is very clear. He should have signed one letter in accordance with the law.
"If we crash out because of what he has done with the letters... without a deal, he bears personal responsibility for that."
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