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 majority of 58.
It 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.
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”.
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.”
European Commission secretary general Martin Selmayr – right-hand-man to president Jean-Claude Juncker – tweeted: “12 April is now the new 29 March Brexit.”