MPs have rejected Theresa May’s EU withdrawal agreement on the day the UK was due to leave the EU, reports BBC.
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.
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."
Responding to the vote, European Council President Donald Tusk tweeted: "In view of the rejection of the Withdrawal Agreement by the House of Commons, I have decided to call a European Council on 10 April."
In a statement, the European Commission said the UK would have to "indicate a way forward" by 12 April "for consideration by the European Council".
"A 'no-deal' scenario on 12 April is now a likely scenario. The EU has been preparing for this since December 2017 and is now fully prepared for a 'no-deal' scenario at midnight on 12 April. The EU will remain united," the statement said.
"The benefits of the withdrawal agreement, including a transition period, will in no circumstances be replicated in a 'no-deal' scenario. Sectoral mini-deals are not an option."