A defiant Prime Minister Boris Johnson said he would not negotiate a further delay to Britain's departure from the European Union after losing a vote in parliament on Saturday that means he is obliged to request a postponement.
Parliament voted 322 to 306 in favour of an amendment put forward by Oliver Letwin, a former Conservative cabinet minister, that withheld support for the Brexit deal Johnson clinched with the EU on Thursday.
According to legislation passed earlier, the vote means Johnson is obliged to write to the EU seeking a delay beyond Britain's scheduled departure date of October 31.
But Johnson has repeatedly said he will not do this and on Saturday he stuck to that line.
"I will not negotiate a delay with the EU and neither does the law compel me to do so," Johnson told parliament.
"I will tell our friends and colleagues in the EU exactly what I have told everyone else in the last 88 days that I have served as prime minister: that further delay would be bad for this country, bad for the European Union and bad for democracy."
The European Commission said Britain must now inform the EU executive of its next steps as soon as possible.
The government will not now hold a vote on its Brexit deal on Saturday as planned. Parliament will debate and vote on Monday on whether to approve Johnson's Brexit deal, the leader of the House of Commons Jacob Rees-Mogg said.
On a day of high drama, lawmakers held the first Saturday sitting since the 1982 Argentine invasion of the Falklands, while hundreds of thousands of people gathered to march on parliament demanding another referendum on EU membership.
Protesters gathered outside the parliament building cheered as lawmakers voted in favour of Letwin's amendment.
The amendment proposed that a decision on whether to back a Brexit deal be deferred until all the legislation needed to implement it has been passed through parliament.
Johnson believes this can be achieved by October 31. Others think it would need a short 'technical' delay in Britain's departure from the EU to pass the legislation.
A law already passed by Johnson's opponents obliges him to ask the EU for a Brexit delay until Jan. 31, 2020 if he could not secure approval for his deal by the end of Saturday, however Britain could still leave on October 31 if parliament manages to pass all the necessary legislation by that date.
Letwin said earlier that he hoped Johnson's deal would succeed, but he wanted "an insurance policy which prevents the UK from crashing out on 31 October by mistake if something goes wrong during the passage of the implementing legislation".
Three years after the country voted 52-48% to leave the European project, many Britons say they are bored with the whole Brexit argument and just want the process to end. But others demonstrating on Saturday remain angry that Britain is leaving the EU and want that reversed.
Hannah Barton, 56, a cider maker from Derbyshire in central England, was draped in the EU flag. "I am incensed that we are not being listened to," she said.
"We feel that we are voiceless. This is a national disaster waiting to happen and it is going to destroy the economy."
Jeremy Corbyn, leader of the main opposition Labour Party, backed a second referendum.
"Voting for a deal today won't end Brexit. It won't deliver certainty and the people should have the final say," Corbyn told parliament.
Brexit "Super Saturday" topped a frenetic week which saw Johnson confound his opponents by clinching a new Brexit deal.
When it comes to a vote in a divided parliament where he has no majority, Johnson must win the support of 320 lawmakers to pass his deal.
If he wins, Johnson will go down in history as the leader who delivered a Brexit - for good or bad - that pulls the United Kingdom far out of the EU's orbit.
Should he fail, Johnson will face the humiliation of Brexit unravelling after repeatedly promising that he would get it done - "do or die" - by October 31.
Johnson won the top job by staking his career on getting Brexit done by the latest deadline of October 31 after his predecessor, Theresa May, was forced to delay the departure date. Parliament rejected her deal three times, by margins of between 58 and 230 votes earlier this year.
He says lawmakers face the option of either approving the deal or propelling the United Kingdom to a disorderly no-deal exit that could divide the West, hurt global growth and bring renewed violence to Northern Ireland.
To win, Johnson must persuade enough Brexit-supporting rebels in both his Conservative Party and the opposition Labour Party to back his deal. His Northern Irish allies and the three main opposition parties oppose it.
In a boost for Johnson, some influential hardline Brexit supporters such as Mark Francois and Iain Duncan Smith have said they will support the deal.
Steve Baker, the head of a hardline Brexit faction in the Conservative Party, has told his European Research Group allies they should vote for Johnson's deal.