Prime Minister Theresa May has written to the European Union to ask for a three month delay to Brexit.
The UK is due to leave the EU next Friday, on 29 March – but Mrs May wants that to be postponed until 30 June.
She says in a letter to European Council President Donald Tusk she needs more time to get her withdrawal deal passed and ratified by Parliament.
The EU says it may not grant an extension beyond 23 May. Any delay must be agreed by all 27 member states.
In a document seen by the BBC, the European Commission says: "Any extension offered to the United Kingdom should either last until 23 May 2019 or should be significantly longer and require European elections.
"This is the only way of protecting the functioning of the EU institutions and their ability to take decisions."
French foreign minister Jean-Yves Le Drian told the French Parliament that unless the UK was able to give "sufficient guarantees" it had a credible strategy, it "would lead to the extension request being dismissed" and the EU "opting for a no-deal exit".
Mr Tusk, who speaks on behalf of the 27 other EU member states, is due to make a statement shortly.
Meanwhile, an urgent debate is under way in Parliament, with Labour pressing for further detail about the PM's intentions and demanding any delay is long enough to allow MPs to "break the impasse and find a way forward".
At a highly charged Prime Minister's Questions earlier, Mrs May said MPs had "indulged themselves on Europe for too long" and voters "deserved better".
She said she had rejected calls for a longer delay to Brexit because she wanted to avoid the UK taking part in European elections in May, which she said would be an "unacceptable" three years after voting to leave the EU.
"It would be a failure to deliver on the referendum decision this House said it would deliver," she told the Commons.
But she added: "As prime minister I could not consider a further delay beyond 30 June."
This was seen by some as an indication that Mrs May would resign rather than seek a further delay.
Labour leader Jeremy Corbyn accused her of leading the UK into "crisis, chaos and division".
"We are still legally due to leave the European Union in nine days' time," he told Mrs May at PMQs.
"Months of running down the clock and a concerted campaign of blackmail, bullying and bribery has failed to convince the House or the country that her deal is anything but a damaging national failure and should be rejected."
He urged the prime minister to meet him later on Wednesday to discuss a "compromise to get through this crisis", a plea ignored by the PM.
The Labour leader will travel to Brussels on Thursday to meet the EU's chief negotiator Michel Barnier. He is also expected to hold talks with several EU 27 leaders.
In her letter to Mr Tusk, the prime minister says she had wanted to hold a Commons vote on her withdrawal agreement this week but had been prevented from doing so by Commons Speaker John Bercow.
But she adds: "It remains my intention to bring the deal back to the House."
If the motion is passed, more time will be needed for Parliament to ratify the deal, she says. There is no mention in the letter of a longer delay, as some had been expecting.
Mrs May, who has been meeting groups of cabinet ministers on Wednesday afternoon, told MPs that an extension beyond 30 June would not "take no-deal off the table" if an agreement had still not been reached.
She has repeatedly insisted the UK would leave the EU on 29 March and she told MPs it had always been her preferred option, but with a withdrawal deal to ensure it was an "orderly" exit.
But she was forced to seek a delay after MPs twice rejected the withdrawal deal she has agreed with the EU by massive margins and voted to reject a no-deal Brexit.
Her plan to hold a third vote on the deal this week were blocked by the Speaker, John Bercow, who said it would break longstanding conventions preventing MPs from being repeatedly asked the same question.
MPs from across the political spectrum lined up to attack Mrs May over her announcement at prime minister's questions.
The SNP's Pete Wishart, who wants another EU referendum, accused her of "caving in to Brexiteers".
He urged her to "develop a backbone and stand up to those who would take this nation to disaster".
Yvette Cooper – one of a string of Labour MPs to call for "indicative votes" on different alternatives to Mrs May's Brexit deal – accused the PM of harming the national interest, adding: "I beg this prime minister to think again."
Mrs May said Commons votes had already been held on different Brexit options and the "one positive thing" that had been agreed was to rule out a no-deal departure on 29 March.
Conservative Brexiteer Peter Bone warned Mrs May she will be "betraying" the public if she continues to seek to delay Brexit.
"If you continue to apply for an extension to Article 50 you will be betraying the British people. If you don't, you will be honouring their instruction.
"Prime minister, it is entirely down to you. History will judge you at this moment. Prime minister, which is it to be?"
Mrs May replied: "I am saying that I think we should look again at being able to leave with a negotiated deal, but in order to do that we need time for this Parliament to ratify a deal, and in order to do that we need an extension until 30 June."