What Happened in Today’s Brexit Vote

British lawmakers on Saturday disrupted Prime Minister Boris Johnson’s carefully choreographed plan to push his Brexit deal through Parliament, voting in favor of an amendment that aims to put off the moment of decision until they have had more time to scrutinize his plan.

The move to postpone the crucial Brexit vote muddled Mr. Johnson’s path to a Brexit deal, though it also could end up increasing the chance that some moderate lawmakers will vote for his deal down the road.

The whiplash developments left Mr. Johnson’s agreement in limbo. He is legally obliged to seek yet another extension for Britain’s departure from the European Union, which he had once vowed never to do.

In fact, after the vote on the amendment, Mr. Johnson vowed not to negotiate with European leaders for a Brexit delay.

“I will not negotiate a delay with the E.U.,” he said, “and neither does the law compel me to do so.”

In the absence of Parliament’s approval for his deal, Mr. Johnson is required by law to send a letter to the European Union by Saturday night asking them to delay the Brexit deadline to avoid a no-deal exit.

But Mr. Johnson refused on Saturday to say he would comply with that law.

“I wish the House to know I’m not daunted or dismayed by this particular result,” the prime minister said. “I will tell our friends and colleagues in the E.U. exactly what I’ve told everyone in the last 88 days: that further delay would be bad for this country, bad for the European Union and bad for democracy.”

It was the latest twist in a debate that has convulsed the country ever since the British public voted in 2016 for a divorce from the European Union.

Crowds of anti-Brexit marchers in Parliament Square erupted in cheers and applause at the news that the amendment had passed.

The amendment essentially turned Mr. Johnson’s up-or-down vote on his deal into a weaker one, saying only that “this House has considered the matter but withholds approval unless and until implementing legislation is passed.”

Lawmakers were worried that, were they to approve Mr. Johnson’s deal on Saturday, hard-line Brexiteer lawmakers would delay passing accompanying legislation next week, pushing Britain out of the European Union without a deal on Oct. 31.

The passage of the amendment means that Mr. Johnson is forced by law to send a letter to the European Union on Saturday night saying that, because he could not pass his deal in time in Britain’s Parliament, he needed an extension — a letter he had been doing everything in his power to avoid sending.

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Even lawmakers who support Prime Minister Boris Johnson’s Brexit deal didn’t trust him or his hard-line Brexit backers, fearing that they might pull a procedural trick to force Britain to crash out of the European Union without a deal.

They also worried that Parliament could approve Mr. Johnson’s deal on Saturday, absolving the prime minister of any obligation to delay the Brexit deadline.

So a former Conservative lawmaker, Oliver Letwin, whom Mr. Johnson had kicked out of the party, put forward an amendment as sort of insurance policy to make approval of the deal conditional on also passing necessary legislation.

In essence, the so-called Letwin Amendment, which was chosen by the speaker of the House of Commons, John Bercow for a vote, aimed to turn Parliament’s up-or-down vote on Mr. Johnson’s deal into a much weaker motion.

Saturday would not be the day that lawmakers would fully endorse or reject the Brexit deal.

Mr. Johnson is now legally obligated to send a letter to the European Union on Saturday to request an extension of the Brexit deadline, currently Oct. 31. The prime minister had been doing everything in his power to avoid sending such a letter.

Now that the amendment has passed, lawmakers get to not only cast a definitive vote on Mr. Johnson’s deal, but also to debate, amend and vote on legislation putting that deal into law.

The Democratic Unionist Party of Northern Ireland, which stridently objects to Mr. Johnson’s deal, had signaled that it would vote for the Letwin amendment. Sammy Wilson, a D.U.P. lawmaker, said that “we would be failing in our duty” if the party did not try to force changes to the Brexit deal.

On a high-wire day in British politics, a crucial question is how the government will respond to the upending of Mr. Johnson’s plan.

British news outlets reported that the government could put forward the legislation accompanying Mr. Johnson’s deal as soon as Monday or Tuesday and push for a quick vote then.

Lawmakers say that working through the Brexit legislation itself, however messy and protracted the process, is the only way to guarantee that pro-Brexit lawmakers, by accident or design, do not let Britain crash out of the European Union without a deal.

In what commentators called the biggest political speech of his life, Prime Minister Boris Johnson argued strenuously in the House of Commons on Saturday that his deal was the best available Brexit deal and that Britain could not waste another day in extracting itself from the European Union.

“Now is the time for this great House of Commons to come together,” he said before the vote on the amendment. Amid shouts from the opposition benches, he added that any further delay to Brexit would be “pointless, expensive and deeply corrosive of public trust.”

Mr. Johnson cast his deal as a fulfillment of decades of conflict in Britain over its place in the European Union. He said it would allow the entire country to benefit from future trade deals and avoid a dreaded hard border on the island of Ireland.

Mr. Johnson’s odds were complicated by the fact that he does not have a working majority in Parliament and has not won a major vote there in the three months he has been in office.

In a striking moment on Saturday afternoon, as the debate dragged on before the vote, Theresa May, Boris Johnson’s predecessor as prime minister, stood up and give an impassioned speech in the House of Commons.

“Standing here, I have a distinct sense of déjà vu,” Mrs. May said to knowing laughter, given that her deal had been rejected in the same chamber three times.

For Mrs. May, it was a dramatic intervention, given that she was showing support for Mr. Johnson, who had often not supported her.

She said it was time for Parliament to vote for a deal on Brexit, having promised to abide by the democratic will of the people.

“If the Parliament did not mean it, then it is guilty of the most egregious con trick on the British people,” Mrs. May said. “You cannot have a second referendum simply because you don’t agree with the results of the first.”

“If you don’t want ‘no deal,’” she declared, “you have to vote for a deal.”

Cheers erupted at from the backbenchers the end of her speech.

It was the most visible appearance by Mrs. May in the nation’s Brexit debate since she stepped down from her job and relinquished leadership of the Conservative Party in the wake of her own stinging defeats.

But it also put her in an awkward position. During her negotiations with Brussels, Mrs. May said that no British prime minister could accept a deal that would keep Northern Ireland in the European Union’s customs territory.

Although Northern Ireland would remain in the United Kingdom’s customs territory under Mr. Johnson’s deal, the arrangement would impose the same customs checks between Britain and Northern Ireland that Mrs. May once ruled out.

Jeremy Corbyn, Britain’s left-wing opposition leader, who spoke after Prime Minister Boris Johnson but before Theresa May in the Commons on Saturday, urged lawmakers to vote against the deal.

“This deal is not good for jobs, damaging to our industry and a threat to our environment and our natural world,” he said. “It should be voted down today by this House.”

He argued that the deal was worse than the agreement reached by Mr. Johnson’s predecessor, Theresa May.

“We simply cannot vote for a deal that is even worse than the House rejected three times,” he said.

Mr. Corbyn argued that the new deal would cost every citizen in the country, on average, more than $2,500 and would lead to “a race to the bottom in regulation and standards.”

Huge crowds of protesters streamed to Westminster on Saturday in a march to demand another referendum on Brexit — a show of defiance as British lawmakers debated a deal outlining the nation’s exit from the European Union.

Organizers of the People’s Vote march said they had drawn about one million people, which would make it one of the largest demonstrations on record in Britain.

“We are now reaching a crucial moment in the Brexit crisis,” the organizers said in a statement. “The government has adopted the slogan ‘Get Brexit Done’ to try and browbeat an exhausted public into accepting whatever botched Brexit Boris Johnson presents to them, but we know this slogan is a lie.”

Outside Westminster on Saturday, Milou de Castellane, 52, who works as a nanny in London, said she had voted to remain in the European Union and would like to have a second referendum or to remain in Europe.

On the coming parliamentary vote on Prime Minister Boris Johnson’s deal, she said: “I hope that the deal will not pass, but I have a sinking feeling that it might. But it cannot just be a rabbit-out-of-a hat scenario. We have to know what is in the deal.

Derek Lancaster, 70, a retired environment agency worker from Preston, in northwestern England, said: “I have a feeling that Boris Johnson’s deal will get voted down, but I think he’s aiming for that. He has done his job and got a deal, even if it does not get approved.”

Mr. Lancaster, a Conservative voter, said: “I am quite happy with no deal. It will be a bit hard for a few months and there will be a few adjustments in business and politics and the way the country is run, but we have got to accept the result of the referendum.”

Three 16-year-olds who attend school together in Oxford had descended on Parliament Square on Saturday. They were 13 when the 2016 Brexit referendum took place and still cannot vote in elections in Britain for another two years.

“We came here today because we want to let our voices be heard; we have not been able to do it any other way,” said Anoushka Nairac, a student at Magdalen College School in Oxford. She added that “we have been living with the consequences” of the referendum.

“My father is an immigrant who set up his own company and provided jobs for citizens,” she said. “It makes me annoyed; people are not looking at the facts.”

She added: “The deal is appalling. They have taken Theresa May’s deal and wrapped it in new packaging. The deal is uncaring about E.U. citizens and the Northern Ireland border. The deal is heartless.”

Michelle and Mike Megan, both 60, have been coming from Newbury to protest outside Westminster for a few days each week since January.

Ms. Megan said: “As a leave voter, we are here to counteract the people’s vote to remain in the E.U. Remainers are asking for a people’s vote, but the people already voted in 2016. We were told it was a once-in-a-generation referendum.”

Ms. Megan added: “So far, Boris Johnson has done a good job. I would never have called myself a Boris fan, but he is now our only hope of getting Brexit done. He has his faults, but so do great leaders in the past.”

Reporting was contributed by Stephen Castle, Mark Landler, Ben Mueller, Marc Santora, Anna Schaverien, Claire Moses, Alan Yuhas and Megan Specia.