Boris Johnson has sent a request to the EU for a delay to Brexit - but without his signature.
The request was accompanied by a second letter, signed by Mr Johnson, saying he believes a delay would be a mistake, BBC reported.
The PM was required by law to ask the EU for an extension to the 31 October deadline after losing a Commons vote.
EU Council President Donald Tusk tweeted that he had received the extension request and would consult EU leaders "on how to react".
Opposition MPs have warned the PM that if he tries to circumvent Parliament's instructions to seek a delay, then he may find himself in the law courts.
Mr Johnson previously said he would "rather be dead in a ditch" than ask the EU to delay Brexit, and the UK would leave on 31 October "do or die".
Hours after losing a crunch vote in a historic Saturday session in the House of Commons, the prime minister ordered a senior diplomat to send an unsigned photocopy of the request for a delay, which was forced on him by MPs last month.
The second letter from Mr Johnson - signed off this time - makes clear he personally believes a delay would be damaging.
It says the government will press on with efforts to pass the revised Brexit deal agreed with EU leaders last week into law, and that he is confident of doing so by 31 October.
A cover note from Sir Tim Barrow, the UK's representative in Brussels, explained the first letter complied with the law as agreed by Parliament.
BBC political editor Laura Kuenssberg described the decision to send three documents as "controversial", predicting "there will be a fight about whether Boris Johnson is trying to circumvent the court".
"This is heading straight for the court, and it may very quickly end up in the Supreme Court," she added.
At the first Saturday sitting in the Commons for 37 years, MPs voted in favour of an amendment withholding approval of Mr Johnson's Brexit deal until all the necessary legislation to completed.
Tabled by Tory MP Sir Oliver Letwin, the amendment was intended to ensure that Mr Johnson would comply with the terms of the so-called Benn Act.
Under that act, passed by MPs last month and which required the PM to seek a Brexit extension, Mr Johnson had until 23:00 BST on Saturday to send a letter requesting a delay.
In a letter to MPs and peers on Saturday evening, he said the EU could reject "Parliament's request for further delay", adding he "will not negotiate a delay".
"I will tell the EU what I have told the British public for my 88 days as prime minister: further delay is not a solution," he said.
The Commons defeat marked a major setback for the PM - although Mr Johnson said he was not "daunted or dismayed" and remained committed to taking Britain out by the end of the month with his "excellent deal".
'Acting like a child'
However, Labour leader Jeremy Corbyn said in a tweet the prime minister had been told to obey the law and "despite his petulant posturing and bluster he finally has - he's asked for an extension".
He told a rally in Liverpool that Labour had "answers" for Mr Johnson's Brexit plan.
"We will challenge them all the way, in Parliament next week, we challenge them in crashing out of the EU," he added.
Today I told the Prime Minister to obey the law and despite his petulant posturing and bluster he finally has - he's asked for an extension.
His damaging deal was defeated today.
Anna Soubry, of Change UK, accused the prime minister of acting like a "truculent child", while Liberal Democrat Christine Jardine MP said Mr Johnson was going against the will of the Commons.
SNP leader at Westminster Ian Blackford said earlier that if Mr Johnson acted as if he was "above the law", he would find himself in court.
Labour's Stephen Doughty told the BBC lawyers would be examining Mr Johnson's actions.
"While he's sent the legally required letter - that's quite frankly the only one... the EU leaders will consider legally - if he's attempting essentially to circumvent or undermine the first one, which he's required to [send] by law, that's a very serious matter."
On the EU side, there has been no official response yet to the contents of the letter, except Mr Tusk confirming he received it.
But the French President, Emmanuel Macron, has already signalled that he believes a new Brexit extension was not good for anyone.
The EU Parliament's Brexit Steering Group will discuss the outcome of the latest UK vote on Monday, said Brexit co-ordinator Guy Verhofstadt.
He tweeted on Saturday: "Whatever happens next, the marches outside the Parliament show just how important a close EU - UK future relationship is."
Jacob Rees-Mogg, leader of the Commons, said the government plans to hold a meaningful vote on Monday - although it is not clear if Commons Speaker John Bercow will allow it.
Mr Johnson has also vowed to press ahead with the legislation to implement the deal next week.
BDST: 1241 HRS,OCT 20, 2019
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