Prime Minister Theresa May faced a confidence vote on Wednesday after MPs overwhelmingly rejected her deal on leaving the European Union, raising fears of a disorderly "no-deal" Brexit.
The humiliating defeat left the process in limbo with 73 days to go before the official departure date and raised warnings of Britain crashing out of the bloc.
May suffered the biggest government defeat in modern British history on Tuesday night when the House of Commons rejected by 432 votes to 202 the withdrawal agreement she struck with Brussels late last year.
The EU immediately warned that the vote raises the risk of a hugely disruptive "no deal" Brexit where Britain could sever ties with its biggest trading partner overnight.
'Time is almost up.'
European Commission chief Jean-Claude Juncker urged London to "clarify its intentions as soon as possible", warning: "Time is almost up."
EU negotiator Michel Barnier said he "regretted profoundly the vote" but that it was "too early to assess all the consequences".
Ireland, the only EU member state with a land border with Britain, said it would now intensify preparations for a "disorderly Brexit".
May struck a conciliatory tone after the vote, promising cross-party talks to try and salvage a workable Brexit deal before returning with a new plan next Monday.
May's own backbenchers rebelled in huge numbers, but the ringleaders said they would vote for her in the confidence vote scheduled for around 1900 GMT, which she is expected to win.
"We are going to vote with the government... of course the Conservatives are going to support the Conservative government," leading Brexiteer Steve Baker told the BBC.
The motion was tabled by opposition Labour leader Jeremy Corbyn, who wants to force a general election.
May's parliamentary allies in Northern Ireland's Democratic Unionist Party (DUP) have led the charge against her Brexit deal, but do not want a Labour government.
However, Britain's newspapers were withering in noting the scale of her defeat, a damning verdict on 18 months of negotiations with the EU.
The top-selling Sun tabloid said May was "crushed", while the Daily Mail said her power was left "hanging by a thread."
One commentator in The Times described her as a "zombie prime minister".
Ahead of the 2016 referendum, most lawmakers — including May —opposed Brexit, which has caused bitter divisions across the nation.
Nearly three years later, they are still to form a majority consensus around any plan, with MPs criticising May's deal both for keeping Britain too closely tied to the EU, and for not keeping it closer.
Former foreign minister and leading Brexit supporter Boris Johnson said Tuesday's vote gave May a "massive mandate" to renegotiate her deal with the EU.
DUP leader Arlene Foster, whose small party props up May's government in the Commons, also called for "fundamental change" to the Brexit deal.
But, speaking ahead of Tuesday's vote, German Foreign Minister Heiko Maas ruled out any substantial changes to the withdrawal agreement — a message reinforced by Juncker.
"It is the only way to ensure an orderly withdrawal of the United Kingdom from the European Union," Juncker said.
After the defeat, French President Emmanuel Macron said "the pressure" was mainly on Britain to find a solution, but suggested there may be room to "make improvements on one or two things."
The defeat blows the starting whistle on a no-holds-barred struggle to direct the course of Brexit. Increasing numbers of pro-European MPs are calling for a second referendum with an option to cancel Brexit.
EU Council president Donald Tusk tweeted after the vote: "If a deal is impossible, and no one wants no deal, then who will finally have the courage to say what the only positive solution is?"
Speculation is also growing on both sides of the Channel that May could ask to delay Brexit, although she denies this.
On Wednesday morning the pound edged down, but mostly held its ground as investors considered the next likely developments in the saga.
"Momentum is shifting away from the harder Brexit route and towards a number of options ranging from postponement and second referendum. That is pound supportive," said Gavin Friend at National Australia Bank.
British businesses urged politicians to unite.
"Financial stability must not be jeopardised in a game of high-stakes political poker," warned Catherine McGuinness, policy chair at the City of London Corporation, the body governing the British capital's massive financial district.