Boris Johnson has asked the EU for a three-month delay to Brexit, which would extend the Article 50 negotiating period to 31 January 2020.
The request to European Council President Donald Tusk was made in an unsigned letter late on Saturday night.
It came hours after MPs voted to withhold their approval for Mr Johnson's Brexit deal unless and until he has passed all necessary legislation to implement it.
The prime minister was legally required to request this delay under the so-called "Benn Act". The legislation was passed by opposition MPs in September to prevent a no-deal Brexit at the end of this month.
On Twitter, Mr Tusk said: "The extension request has just arrived. I will now start consulting EU leaders on how to react."
Mr Johnson - who has previously said he would rather die in a ditch than delay Brexit again - has rung European leaders to distance himself from the request, describing it as "parliament's letter, not my letter".
The PM has also sent a separate note to Mr Tusk, obtained by Sky News, in which he urged the EU not to grant the extension.
Warning that pushing the deadline back would be "deeply corrosive", Mr Johnson wrote: "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."
Next week, the prime minister will introduce the legislation needed to put his Brexit deal into UK law, in the hope that a majority of MPs give their backing to his agreement so he can still meet his pledge to take the country out of the EU on 31 October.
On what had been dubbed "Super Saturday", the House of Commons voted 322 to 306 in favour of an amendment - proposed by former Conservative minister Sir Oliver Letwin - to the motion on the prime minister's Brexit deal.
Sir Oliver had said his amendment was an "insurance policy" to prevent Britain "crashing out" of the EU without a deal on 31 October.
Mr Johnson wrote to all MPs and peers after the Commons vote to state he will "not negotiate a delay with the EU", while he will also tell Brussels that "further delay is not a solution".
He also warned the EU could possibly "reject parliament's request for further delay, or not take a decision quickly".
The prime minister suggested, in those circumstances, MPs could yet be faced with a choice between his deal or a no-deal Brexit.
In a conversation with Mr Johnson, French President Emmanuel Macron "signalled a delay would be in no one's interest".
Irish foreign minister Simon Coveney reiterated Dublin's stance that a Brexit delay is "preferable" to a no-deal Brexit, although he also stressed all other EU member states would need to unanimously back an extension for it to be granted.
"Any one prime minister can prevent that and I think the EU wants to see certainty and an end to endless negotiation and speculation, so I think a request for an extension is not straightforward," he added.
The DUP gave their backing to Sir Oliver's amendment. In the days leading up to Saturday's sitting, the Northern Irish party had expressed fierce opposition to the prime minister's deal.
Eight of the 21 former Conservative MPs the prime minister withdrew the whip from last month - including ex-cabinet ministers Philip Hammond, David Gauke and Justine Greening - also backed Sir Oliver's amendment.
They were among a total of 10 former Tory MPs, who now sit as independents, to support the amendment.
Six Labour MPs defied their party's orders and voted against the amendment.
There were cheers from among the tens of thousands of people at a People's Vote rally in Westminster, who want a second EU referendum, as the vote result was announced.
Speaking in the Commons after his defeat, Mr Johnson told MPs the so-called meaningful vote on his Brexit deal had "effectively been passed up because the meaningful vote has been voided of meaning".
But the prime minister said he was not "daunted or dismayed" by what he described as a "pretty close" result.
Mr Johnson expressed his hope the EU "will not be attracted" to granting an Article 50 extension.
Leader of the House of Commons, Jacob Rees-Mogg, revealed the government will attempt to stage another meaningful vote on the Brexit agreement on Monday.
But Commons Speaker John Bercow hinted he could not allow such a vote as its "apparent purpose... is to invalidate or obviate" Sir Oliver's amendment.
It has been suggested that if the government did win a meaningful vote on the Brexit deal on Monday, the prime minister could withdraw his request for an Article 50 extension or hope it persuades the EU not to grant a delay.
Next week, the government will also introduce the Withdrawal Agreement Bill needed to put Mr Johnson's Brexit deal into law.
On Twitter, Labour leader Jeremy Corbyn wrote: "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."
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