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.
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".
British PM faces defeat in historic Brexit deal vote
British Prime Minister Theresa May faces crushing defeat in a historic vote in parliament on Tuesday over the Brexit deal she has struck with the European Union, leaving the world's fifth biggest economy in limbo.
With just over two months to go until the scheduled Brexit date of March 29, Britain is still bitterly divided over what should happen next and the only suspense over the vote is the scale of May's defeat.
The British leader's last-minute appeals to MPs appear to have fallen on deaf ears and how much she loses by could determine whether she tries again, loses office, delays Brexit - or if Britain even leaves the EU at all.
"When the history books are written, people will look at the decision of this house... and ask: did we deliver on the country's vote to leave the European Union," May asked MPs on the eve of the vote, expected after 1900 GMT.