MPs have rejected Theresa May’s EU withdrawal agreement on the day the UK was due to leave the EU.
The government lost by 344 votes to 286, a margin of 58, and means the UK has missed an EU deadline to delay Brexit to 22 May and leave with a deal.
The prime minister said the UK would have to find “an alternative way forward”, which was “almost certain” to involve holding European elections.
Labour’s Jeremy Corbyn said “this deal now has to change” or the PM must quit.
Meanwhile, thousands of Leave supporters gathered outside Parliament to protest against the delay to Brexit, bringing traffic to a standstill.
Mrs May now has until 12 April to seek a longer extension to the negotiation process to avoid a no-deal Brexit on that date.
With a clear majority in the Commons against a no-deal Brexit, and with MPs holding more votes on alternative plans on Monday, Mrs May said that the UK would have to find “an alternative way forward”.
The prime minister said that the outcome was “a matter of profound regret”, adding that “I fear we are reaching the limits of this process in this House”.
Downing Street said it was still not an “inevitability” that the UK would have to take part in elections to the European Parliament in May.
It is highly likely that at least for another couple of weeks, Theresa May will look through every nook and cranny in Parliament to see if there is a way for her deal to pass through – somehow.
But that’s a decision taken in the bunker, and the walls are closing in.
There is little reason tonight to think that, in the end, the burning core of Euroscepticism in the Tory Party will ever accept her deal.
There are few signs that any more than a handful of Labour MPs are really going to take the plunge and ultimately walk through the same lobbies as Theresa May, and Boris Johnson and Iain Duncan Smith.
The prime minister concluded today that our political process is reaching its limits.
But maybe soon it will be her leadership, her deal, that has passed its limits.
A No 10 source indicated that the prime minister would continue to seek support in the Commons for her deal.
“Clearly it was not the result we wanted. But, that said, we have had a number of senior Conservative colleagues who have felt able to vote with the government today. They have done so in higher numbers than previously,” the source said.
“Clearly there is more work to do. We are at least going in the right direction.”
Downing Street said Mrs May would continue to talk to the Democratic Unionist Party about more reassurances over the Irish backstop, which it says risks splitting Northern Ireland from the rest of the United Kingdom.
But the DUP’s leader at Westminster leader, Nigel Dodds, told the BBC’s Newsnight political editor Nick Watt: “I would stay in the European Union and remain, rather than risk Northern Ireland’s position. That’s how strongly I feel about the Union.”