Britain's parliament on Tuesday resoundingly rejected Prime Minister Theresa May's Brexit deal, triggering a no-confidence vote in her government and plunging its plans to leave the EU into further disarray.
MPs voted 432 to 202 against May's plan for taking Britain out of the European Union, the biggest parliamentary defeat for a government in modern British political history.
The EU immediately warned that it heightened the risk of a "no deal" Brexit - an outcome that could disrupt trade, slow down the UK economy, and wreak havoc on the financial markets, where London is a global player.
"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?" EU president Donald Tusk tweeted.
The government of Ireland - the only EU member state with a land border with Britain - said it would now intensify preparations to cope with a "disorderly Brexit".
Moments after the outcome in parliament, which was met with huge cheers by hundreds of anti-Brexit campaigners who watched the vote on big screens, opposition Labour leader Jeremy Corbyn submitted a motion of no-confidence in May's government, calling her defeat "catastrophic".
The vote is expected on Wednesday at 19:00 GMT.
May sought to strike a conciliatory tone, telling MPs they had the right to challenge her leadership and promising to hold more talks to salvage a workable deal by the rapidly approaching March 29 Brexit deadline.
She promised to hold discussions with MPs from across parliament to identify ideas "that are genuinely negotiable and have sufficient support in this House".
"If these meetings yield such ideas, the government will then explore them with the European Union."
Downing Street said May will come back to parliament with a new Brexit proposal on Monday.
Most lawmakers opposed Brexit, as did May herself and leading members of her government, ahead of the June 2016 referendum on EU membership, which has caused bitter divisions across the island nation.
Now, nearly three years after the fateful referendum and with just over two months to go, Britain still cannot decide what to do.
With their nation's fate hanging in the balance, noisy supporters and opponents of Brexit, some banging drums and others driving floats with huge dolls mocking top UK politicians, rallied outside the ancient parliament building in London.
"It could end up being the day that will lead to us leaving with no deal!" said 25-year-old Simon Fisher, who backs a swift and sharp break with the EU.
A much larger rally nearby in support of a second referendum turned Parliament Square into a sea of EU flags.
May's defeat had long been factored in by the markets and the pound rebounded from session lows against both the dollar and euro after the vote on the back of hopes that a no-deal Brexit could be avoided.
But British businesses sounded a note of alarm, urging politicians to unite at a time of national crisis.
"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.
"Every business will feel no deal is hurtling closer. A new plan is needed immediately," said Carolyn Fairbairn, head of the Confederation of British Industry business lobby.
'Warm words not enough'
May made it her mission to carry out the wishes of voters after she became premier in July 2016, putting aside her own initial misgivings and stating repeatedly that "Brexit means Brexit".
But facing a heavy drubbing, she decided to postpone a parliamentary vote in December on the Brexit deal in the hope of winning concessions from Brussels - and that a Christmas break would change lawmakers' minds.
Hardline Brexiteers and Remainers oppose the agreement for different reasons and many fear it could lock Britain into an unfavourable trading relationship with the EU.
Criticism of the deal is focused on an arrangement to keep open the border with Ireland by aligning Britain with some EU trade rules, if and until London and Brussels sign a new economic partnership which could take several years.
Arlene Foster, head of Northern Ireland's Democratic Unionist Party upon which May relies for her parliamentary majority, said May needed to win binding concessions from Brussels to secure her vote.
"Reassurances whether in the form of letters or warm words, will not be enough," said Foster.
"The prime minister must now go back to the European Union and seek fundamental change to the Withdrawal Agreement."
Speculation is growing on both sides of the Channel that May could ask to delay Britain's divorce from the EU after almost half a century of membership.
But a diplomatic source told AFP any extension would not be possible beyond June 30, when the new European Parliament will be formed.
The withdrawal agreement includes plans for a post-Brexit transition period until a new relationship is drawn up, in return for continued budget contributions from London.