Prime Minister Theresa May's government was defeated on Wednesday, when lawmakers forced through changes to its Brexit blueprint that ministers said could endanger Britain's departure from the European Union.
The Parliament, including a dozen members of her Conservative Party, voted 309 to 305 in favour of an amendment to demand Parliament pass a separate bill to approve any final deal with the EU. Lawmakers want more say over the final exit deal.
The vote came after six days of debate in Parliament ranging from the legal minutiae of Brexit to the gaping differences between so-called Remainers and Leavers.
Up until the last minute of an often bitter debate in Parliament, May's team tried to convince lawmakers in her party to give up their demands and side with a government, fearful that the move will weaken its hand in tough Brexit negotiations.
MPs are debating the EU withdrawal bill, which will repeal the 1972 legislation binding Britain to the EU and copy existing EU law into domestic law to ensure legal continuity after the so-called Exit Day on March 29, 2019.
May due in Brussels on Thursday
In focus on Wednesday was an amendment put forward by Conservative lawmaker and former attorney general, Dominic Grieve, who wants Parliament to have a meaningful vote on any deal before it is finalized.
"There is a time for everybody to stand up and be counted," Grieve told Parliament earlier, criticizing some fellow members of the Conservative Party for calling him a traitor over his decision to vote against the government.
He said he had never "rebelled" against his government in over 20 years, invoking a Winston Churchill quote to characterize himself as a "party man."
May warned that the amendment could push the complex task of transposing EU law legislation right to the end of the two-year negotiation period triggered earlier this year.
She is scheduled to meet with EU leaders on Thursday in Brussels.
"We are disappointed that Parliament has voted for this amendment despite the strong assurances that we have set out," the government said in a statement.
"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."
In the European parliament, which must also ratify any withdrawal treaty with Britain, its Brexit co-ordinator cheekily tweeted that his British counterparts had "taken back control" — a reference to the catchphrase of pro-Brexit campaigners.
"A good day for democracy," added Guy Verhofstadt.
'Humiliating loss of authority'
Grieve dismissed a last-minute pledge by justice minister Dominic Raab for government to write the promise of a meaningful vote into law later on its journey through both houses of Parliament as coming "too late."
Pro-Brexit lawmakers fear the amendment could force Britain to weaken its negotiating stance by offering Parliament the opportunity of forcing ministers back to the negotiating table if it feels any final deal is not good enough.
Raab said that could convince the EU that Britain would not walk away from a bad deal. "Actually if that looked likely we'd end up with worse terms, and we'd be positively incentivizing the EU to give us worse terms," he told Parliament.
The process has highlighted May's position of weakness.
In June, she gambled on a snap election to strengthen her party's priority in the 650-seat Parliament but instead bungled her campaign and ended up with a minority government propped up by the 10 votes of a small, pro-Brexit Northern Irish party. Since then she has struggled to assert her authority over a Conservative Party which is deeply divided over the best route out of the EU.
But pushing her out could only serve to send more public support to the Labour Party, who made considerable gains in June's election.
Labour's Jeremy Corbyn, the Opposition leader, was quick to pounce on Wednesday's result.