Rebel Tory MPs have joined forces with Labour to inflict a fresh blow on Theresa May’s government in a Commons Brexit vote.
It means the government will have to return to Parliament with fresh plans within three days if Mrs May EU withdrawal deal is rejected by MPs next week.
It could also open the door to alternatives, such as a referendum.
The government lost by 11 votes, with 297 MPs voting with them and 308 against.
It comes ahead of five days of debate on the PM’s Brexit deal.
Commons Speaker John Bercow faced an angry backlash from some Conservative MPs over his decision to allow MPs to vote on the issue.
The MPs claim Mr Bercow broke Commons rules and ignored the advice of his own clerks.
Mr Bercow said he had made an “honest judgement” after consulting his clerks and said that if people wanted to vote against the amendment they could.
MPs, including Commons leader Andrea Leadsom, challenged his ruling in a series of points of order after Prime Minister’s Questions.
They argued that the business motion, tabled by the government, was not amendable and said the Speaker was breaking with precedent.
Mr Bercow, who said he was standing up for MPs and Parliament, said MPs would not be able to debate the rebel Tory amendment but would be able to vote on it.
He said he had made his decision after speaking to his clerks but rejected calls from Andrea Leadsom to publish the advice he had received.
The clashes in the Commons came ahead of Theresa May launching a fresh push to convince MPs to back her Brexit deal at the start of five days of debate.
The prime minister cancelled a vote on her deal last month at the last minute to avoid a humiliating defeat.
She is hoping new proposals on Northern Ireland will change enough MPs’ minds to save the deal.
But the DUP have already rejected the plans, saying they are “cosmetic” and “meaningless”.
MPs, headed by former Tory ministers Oliver Letwin and Dominic Grieve, helped inflict a defeat on the government on Tuesday on an amendment aimed at making it more difficult to leave the EU without a deal.
Labour has, meanwhile, said it will table a motion of no confidence in the government if Mrs May’s deal is voted down next week.