LONDON — Just as Britain appeared on the cusp of a history-making, up-or-down vote on its long-delayed departure from the European Union, the British Parliament inflicted another humiliating defeat on Prime Minister Boris Johnson on Saturday, adopting a measure that seems likely to delay a vote on his Brexit deal beyond Oct. 31 — something he said he would never accept.
The turbulent events left Mr. Johnson’s agreement in limbo, and threw British politics once again into chaos, with any number of possible outcomes — a no deal exit, a second referendum, a general election, or Mr. Johnson’s deal. The only sure result was continuing frustration and confusion among the British public.
Mr. Johnson is legally obliged to seek yet another extension for Britain’s departure, though he said he would rather “die in a ditch” than do so. He said in Parliament on Saturday that he would not “negotiate” an extension with the European Union, suggesting he would request one and leave the decision on its length to the leaders of the bloc.
It was the latest twist in a trauma that has convulsed the country ever since the British public voted in 2016 for a divorce from the European Union.
On a dramatic day in which lawmakers debated while enormous crowds of anti-Brexit protesters marched outside the Houses of Parliament, Mr. Johnson implored lawmakers to approve his agreement, which would pave the way for Britain to leave the European Union at the end of the month.
The prime minister argued that it was the best deal Britain could hope to strike with Europe — one that, in his telling, would position the country for a thriving future as an agile, free agent in the global economy — and that any further delay would be “pointless, expensive and deeply corrosive of public trust.”
“As someone who passionately believed we had to go back to our European friends to seek a better agreement,” Mr. Johnson said, “I must tell this House that with this deal, the scope for future negotiations has run its course.”
Instead, by a vote of 322 to 306, they passed a last-minute amendment, brought by Oliver Letwin, an expelled member of Mr. Johnson’s Conservative Party, that would delay a vote on the agreement until Parliament had passed the detailed legislation that enacts it.
Mr. Letwin, a veteran lawmaker, was purged from the Conservative Party last month for supporting a law intended to prevent Britain leaving the European Union without any agreement, which many see as risking a disorderly, economically damaging rupture.
The amendment brings into play that law to prevent a no-deal Brexit, pressuring Mr. Johnson to request another delay from the European Union — something he said he would rather be “dead in a ditch” than do.
Mr. Letwin, who actually supports Mr. Johnson’s plan, argued that his amendment was simply a safety net that prevented pro-Brexit hard-liners from sabotaging the implementing legislation and, in the ensuing political vacuum before the Oct. 31 deadline, engineering the no-deal rupture that some want.
But some opponents of the Brexit blueprint supported the Letwin amendment too. For Mr. Johnson, who has staked his claim to 10 Downing Street on delivering the withdrawal, the amendment was another in a long series of setbacks in Parliament, preventing him from forcing lawmakers into a binary decision on whether to support his plan or not.
The government plans to press ahead with its Brexit blueprint, forcing another critical vote on Tuesday. But that could also present opponents with the opportunity to try to amend his plan.
Assuming that Mr. Johnson does request another Brexit delay, as he is obliged to do, the European Union would have to decide whether to grant it and, if so, for how long. European leaders would calculate whether to grant a brief delay of a few more weeks to resolve the technical details, or a longer delay to allow a general election or perhaps a second referendum.
Meeting on a Saturday for the first time since the Falklands War in 1982, members of the House of Commons rose, one after the other, to fervently endorse or reject Mr. Johnson’s deal. The debate seemed to be ultimately less about the details of the plan, with its fiendishly complicated arrangements for trade with Northern Ireland, than about whether Britain could finally put Brexit behind it.
Opponents of the plan accused Mr. Johnson of negotiating a shoddy deal that would leave a post-Brexit Britain vulnerable to predatory trade deals with other countries, not least the United States.
“This deal would inevitably lead to a Trump trade deal, forcing the U.K. to diverge from the highest standards and expose our families to chlorine-washed chicken and hormone-treated beef,” said the leader of the Labour Party, Jeremy Corbyn, referring to fears of chemically-treated imports from the United States.
For Mr. Johnson, 55, a flamboyant politician and former mayor of London who has been in office since July, it was a crucial moment. He spoke with a tone of gravity and conciliation that contrasted starkly with the inflammatory language he has used during previous parliamentary debates over Brexit.
A victory could fuel Mr. Johnson’s Conservative Party in the general election he is likely to call for the coming weeks. A further delay could paint Mr. Johnson as a leader stymied by Brexit, much like his predecessor, Theresa May, who lost three thumping votes in Parliament on her Brexit agreement.
For Mr. Johnson, the arithmetic was always going to be daunting. To win, he would need to cobble together a complex, contradictory collection of lawmakers, including converts from the opposition Labour Party, hard-line Brexiteers who rejected Mrs. May’s agreement, and Conservative Party exiles whom he purged after they voted for the amendment that defanged his threat to leave the European Union, even without a deal, on Oct. 31.
Mr. Johnson’s deal differs from those of his predecessor, Theresa May, primarily in its treatment of the border issue in Northern Ireland. Needing to avoid physical border checks, Mrs. May opted to keep the entire United Kingdom in the European Union’s customs union, which was unacceptable to hard-line Brexiteers.
Mr. Johnson sought to satisfy them by keeping Northern Ireland subject to the bloc’s rules in a practical sense, but legally outside it with the rest of Britain.
His deal is at the extreme end of divorce settlements that Britain could have negotiated with Europe. It commits the country to very little alignment with the European Union on trade or regulations, turning its back on much of the web of rules that critics in Britain found stifling or a threat to their sovereignty.
By keeping the European Union at arm’s length, Mr. Johnson and his lieutenants contend, Britain will be unshackled and can set out to transform itself into an agile, lightly regulated competitor in the global economy — or “Singapore-on-Thames,” to use a phrase coined by Brexit evangelists.
To do that, however, Britain must first negotiate new trade agreements with dozens of parties, including the European Union and the United States, a painstaking process that could take several more years. And Mr. Johnson’s plan allows for only a standstill period ending in 14 months time, though this could be extended for a maximum of two years.
Leaving the European Union legally does not end the Brexit drama; it merely brings down the curtain on Act One.
The debate on Saturday came after more than three tumultuous years of division and discord over Brexit, an ordeal that has shaken British politics and tested traditional loyalties, both among lawmakers and voters.
In 2017, Mrs. May called an election betting that she could persuade Britons to give her a big majority in Parliament to negotiate a Brexit accord. That proved a fatal error when she lost her majority and with it, much of her authority within the governing Conservative Party.
Though she later succeeded in negotiating a Brexit deal, she failed three times to get it through the House of Commons and was ultimately forced to request two Brexit delays. Even before that humiliation, her enemies were circling — not least Mr. Johnson, who resigned from her cabinet after complaining that her deal would make Britain a vassal state of the European Union.
That helped feed a narrative that has polarized British politics, with many supporters of Brexit moving toward a more brutal rupture with the European Union than its proponents suggested in the 2016 referendum.
At the same time, opponents of Brexit became less inclined to settle on a compromise that they saw as the worst of both worlds. Voters increasingly came to identify themselves more as “leavers” or “remainers” than by traditional loyalty to any party.
Facing competition from the Brexit Party, led by Nigel Farage, the Conservatives have now embraced a hard-line form of exit, a transition that gained momentum last month with the purge of 21 Conservative rebels, including Mr. Letwin.
The Labour Party still says it wants to negotiate a different, softer Brexit deal, and would put that to a referendum, with remaining in the European Union being the alternative. The smaller and more pro-European Liberal Democrats say they would stay in the bloc without holding a second vote.
But while political sentiment has fled the center ground, there is a growing sense of exhaustion among many voters about the endless haggling over Brexit in Parliament.
That has proved a powerful weapon for Mr. Johnson, who has argued that he would “get Brexit done” — even if the reality is that Britain’s legal departure from the European Union is only a stage in a much longer process.