Exactly 1,000 days on from Britain's seismic 2016 referendum vote to leave the European Union, a divorce deal negotiated with the EU by May is blocked in parliament and uncertainty grips the country.
May told a stormy session of parliament that she had written to EU President Donald Tusk "informing him that the UK seeks an extension... to June 30".
She was confronted with a barrage of criticism from Brexit hardliners who wanted her to stick to the March 29 departure date and pro-EU lawmakers who want a long delay that could allow for an overhaul of the strategy.
Faced with the potentially catastrophic impact of Britain leaving its biggest trading partner after 46 years with no agreement in place on March 29, May said she would try one last time to pass her deal through parliament.
If she fails a third time, May said parliament would have to decide what happened next -- but hinted that her own future was on the line.
"As prime minister, I am not prepared to delay Brexit any further than June 30," she said.
Delaying any longer would require Britain to hold European Parliament elections at the end of May, which she said would be "unacceptable".
May will travel to Brussels on Thursday for an EU summit where she will put her request in person to the other 27 EU leaders, who must unanimously approve any delay.
But European Commission President Jean-Claude Juncker said it was unlikely they would reach a decision without a clear signal from London on what MPs want, suggesting another meeting might be needed next week.
The commission, the EU's executive arm, said the June 30 date carried "serious legal and political risks".
Spokeswoman Mina Andreeva said Juncker in a phone-call on Wednesday "warned" May against proposing an extension date that was after European elections.
'End of the road'
After almost two years of talks, May and the EU struck a divorce agreement late last year, but MPs have overwhelmingly rejected it twice.
Her original plan to hold a third vote this week was dramatically blocked by Commons speaker John Bercow, who said May could not keep bringing the deal back to MPs without changes.
European leaders are now demanding Britain tells them clearly what it wants.
"As long as we don't know what Britain could say yes to, no decision can be taken on our side either," Juncker told German broadcaster Deutschlandfunk.
He said the deal agreed was the final offer, warning: "Nothing more can be done, we've reached the end of the road.
"If other scenarios are being discussed, we'd need a new road."
Juncker said May needed to present Brussels with an agreement approved by parliament as well as "a clear timetable".
Given that this was unlikely before the summit's start on Thursday, "we will probably need to meet again next week".
The pound fell sharply on the news losing about half a pence in the space of 10 minutes after May announced the delay, bringing its loss for the day to around one per cent
Brexit-backing British newspapers echoed the general sense of frustration.
The Daily Mail's editorial read: "1,000 days of incompetence, betrayal and now national humiliation... how did victory turn to this?"
In the House of Commons, opposition Labour leader Jeremy Corbyn blamed the prime minister's "incompetence, failure and intransigence", saying she refused to accept her deal had failed.
May had previously warned MPs to expect a lengthy Brexit delay if her deal did not pass before this week's EU meeting.
But she opted for a short option after Brexit-backing ministers reportedly said they would not accept a long extension.
This is despite her own deputy, David Lidington, warning that a request for a short delay would likely be rejected by the EU, and could increase the chance of a "no deal".
"When will she develop a backbone and stand up to those who would take this nation to disaster?" Scottish National Party (SNP) MP Pete Wishart asked.
The pound plunged on currency markets after the announcement, as the European Commission advised EU leaders that a shorter delay to May 23 or a much longer one until at least the end of 2019 would be preferable.