British lawmakers said no to Prime Minister Theresa May for a third time, sending her Brexit deal down to defeat on Friday.

The prime minister's stripped-down version of her twice-defeated Brexit deal lost on Friday by 58 votes — 344 to 286, despite May's promise to resign if the plan passed.

A few Brexit-backing Conservatives who rejected the deal in the first two meaningful votes, switched sides and supported today's vote.

But Labour Party members wouldn't budge, nor would the Democratic Unionist party's 10 MPs.

Afterwards May told MPs: “The implications of the house's decision are grave,” and added: “I fear we are reaching the limits of this process in this house.”

She said MPs would debate alternatives to the political declaration next week, but she gave little clue as to how she planned to proceed.

“This government will continue to press for the orderly Brexit that the result of the referendum demands,” she said.

Jeremy Corbyn responded by calling on the prime minister to step down and trigger a general election.

Steve Baker, the deputy chair of the strongly pro-Brexit European Research Group – many of whose members changed sides to back the deal – also called for May to go.

“This must be the final defeat for Theresa May's deal.

“It has not passed. It will not pass. I regret to say it is time for Theresa May to follow through on her words and make way so that a new leader can deliver a withdrawal agreement which will be passed by parliament.”

The vote was held on the day when Britain was meant to be leaving the European Union.

It was not a third “meaningful vote”, as MPs were only asked to consider the withdrawal agreement, which includes the controversial Irish backstop and secures EU citizens' rights and the post-Brexit transition period.

Under the deal agreed by EU leaders in Brussels last week, Brexit was to be delayed until 22 May if the prime minister could win parliament's backing for the withdrawal agreement this week.

Instead, she will now have to return to Brussels before 12 April to ask for a longer delay – requiring Britain to hold European elections in May – or accept a no-deal Brexit.