LONDON — Bipartisan talks on extricating Britain from the European Union collapsed on Friday, when the opposition Labour Party pulled out, ending the latest attempt to salvage the beleaguered Brexit process and leaving it in a familiar state of deadlock.
In a letter to Prime Minister Theresa May, the Labour leader, Jeremy Corbyn, wrote that the negotiations “have now gone as far as they can.”
For weeks, discussions between ministers in the Conservative government and senior Labour figures had failed to break the Brexit impasse, which has wrecked the leadership of Prime Minister Theresa May and provoked a sharp backlash against both major parties from disenchanted voters, who vented their anger in recent local elections.
Mrs. May has been unable to unite her party behind the Brexit agreement she negotiated with the European Union, leading her to turn to Labour for support. But that has produced nothing except increasingly urgent calls from fellow Conservatives for her to step down.
On Thursday, she announced that she would set out a timetable for her departure next month, and Conservatives whose Brexit views are even further from Labour’s are vying to succeed her.
While the talks have been conducted in good faith, “we have been unable to bridge important policy gaps between us,” Mr. Corbyn wrote in his letter. “Even more crucially, the increasing weakness and instability of your government means there cannot be confidence in securing whatever might be agreed between us.”
The breakdown came as little surprise, since both parties are badly split on Brexit, and neither leader has much room to maneuver. Many Conservative lawmakers were angry about the talks with Labour, and many Labour lawmakers had voiced discontent at taking part in negotiations designed to help salvage Mrs. May’s tenure by getting Brexit over the line.
Mrs. May blamed the opposition’s divisions for the failure of the talks. The problem, she said at a campaign stop in Bristol, was that Labour could not decide “whether they want to deliver Brexit or hold a second referendum.”
The collapse is yet another setback to a prime minister who had already suffered a remarkable string of defeats and has been hounded by members of her own party to leave. Parliament has voted three times to reject her Brexit plan, which would keep Britain closely tied to the European Union at least until the end of 2020 but then extract it from the bloc’s main economic structures. She said earlier this week that she would attempt a fourth vote on her plan early next month.
Mrs. May had hoped to lure Labour with the prospect that Britain could stay — at least temporarily — in a type of customs union with the bloc, thereby eliminating the need for tariffs and many border checks on goods flowing between Britain and continental Europe.
But that did not prove enough to tempt Labour, which is in favor or retaining closer ties to the European Union to protect the economy. It had argued that Mrs. May had not offered enough concessions and that, in any case, her successor might be a hard-line Brexit supporter who would tear up any agreement.
Next week Britons will vote in elections to the European Parliament on May 23, almost three years after they voted in a referendum to quit the bloc.
Mrs. May now has to gird herself for the European elections next week hat could prove disastrous for both the Conservatives and Labour. One recent poll has them trailing both the Brexit Party, led by the hard-line Brexiteer Nigel Farage, and the anti-Brexit Liberal Democrats.
The government is expected to try to seek a consensus in Parliament on the way ahead by holding votes on different options, but previous efforts to do so have failed.
Mrs. May wants the Labour Party to agree to abide by the outcome of such votes, but there is little incentive for the opposition to sign up to an outcome that it cannot control. And, without the support of Labour, that process looks doomed to leave Brexit at an impasse.
However, pressure on Mrs. May and Mr. Corbyn to reach a deal increased earlier this month, following local elections that were bad for both parties. While the Conservatives lost 1,300 seats in local municipalities, Labour failed to take advantage, shedding around 80 itself.
Though Mr. Corbyn, a lifelong Euroskeptic, argued that a deal needed to be reached, his party’s divisions have made that outcome difficult. A very large faction in the Labour Party opposes Brexit and wants a second referendum on any deal, so any agreement with the Conservatives would have risked straining loyalty to a breaking point.