Theresa May lost a key vote on the Brexit deal on Wednesday, giving British lawmakers a legal guarantee of a vote on the final deal agreed with the EU.
British MPs voted 309 to 305 in favour of the amendment to the government's EU withdrawal bill after a group of politicians from the ruling Conservative party rebelled against the Prime Minister, voting alongside opposition party MPs.
Mrs May had warned parliament that voting for the amendment would put the chances of achieving a smooth Brexit at risk.
"We are disappointed that parliament has voted for this amendment despite the strong assurances that we have set out," the government spokeswoman said in a statement.
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"This amendment does not prevent us from preparing our statute book for exit day. We will now determine whether further changes are needed to the (EU withdrawal) bill to ensure it fulfils its vital purpose."
Mrs May had tried to convince politicians in her party to vote with the government against the amendment but in the end 11 rebelled, including former Education Secretary Nicky Morgan.
The former minister tweeted after the vote: "Tonight Parliament took control of the EU Withdrawal process."
While Jeremy Corbyn, the leader of the main opposition party, Labour, said the defeat was "a humiliating loss of authority" for the prime minister.
The amendment was authored by former attorney general Dominic Grieve, who told ministers they had "run out of road" adding that he was putting his country before his party.
Mr Grieve, who said this was only the second time he had rebelled in over 20 years as an MP, told the Commons he was gravely concerned the Brexit legislation could become "very worrying tool of executive power".
The narrow defeat has exposed how fragile Mrs May’s government is, having lost her parliamentary majority in June’s election.
The prime minister will head to Brussels on Thursday for an EU summit in which European leaders are expected to give the green light for Brexit talks to move on to discussing trade.