BRUSSELS — Wistful Europeans once hoped Britain would reverse itself on Brexit, abandoning its divorce from the European Union in favor of kissing and making up.

As months of departure negotiations dragged into years, devoted Anglophiles in Europe fantasized about a second referendum in which all was reconsidered, then forgiven. They seized on the ups and downs of the Liberal Democrats in Britain, the party that is most unabashedly pro-European. They cheered as Labour Party activists overwhelmingly supported a do-over referendum at their party conference last fall.

European Council President Donald Tusk said in April that he had been warned against dreaming Brexit could be reversed. Still, he resisted, saying, “At least I will not stop dreaming about a better and united Europe.”

But some of those who once wished for a change of heart now just want to finalize the breakup and move on.

E.U. leaders would almost certainly agree to a delay beyond the Oct. 31 Brexit deadline, if Prime Minister Boris Johnson asks for one — as a new law passed by the British Parliament requires him to do. The Europeans do not relish the prospect of an abrupt and economically destabilizing no-deal Brexit, preferring an agreement to manage the withdrawal and ease the transition to new trade terms.

At the same time, though, E.U. negotiators are eager to usher Britain out. Policymakers in Brussels and in capitals throughout Europe worry about the consequences of continued uncertainty — and some say Britain is so poisoned on E.U. issues that it might be more destructive inside the bloc than outside of it.

“Immediately after the referendum, among the pro-Europeans in the European institutions, there was the wish that, over the course of time, there would possibly be a reversal,” said Alain Lamassoure, a longtime French member of the European Parliament who retired in June. “But after three years, after all these appalling, ridiculous, dramatic things in the House of Commons, there is a very wide sentiment whereby too much is too much. Now, it’s too late. And it would be better to put an end to this drama.”

Lamassoure said his own reversal occurred in January as he watched British lawmakers vote down the withdrawal deal negotiated between E.U. leaders and then-Prime Minister Theresa May by 432 to 202 — a defeat with little precedent in modern British history.

“It was a shock for all of us, because we overestimated the spirit and the strength of the British parliamentary system,” he said.

Until Britain formally leaves the E.U., it retains the right to cancel the divorce notification — the Liberal Democrats are campaigning on just such a proposal. Legally, relations would return to the status quo. So some European policymakers like to game out the result they once dreamed about.

But what they see now gives them a headache. Britain remains profoundly split on the question of its relationship with Europe. Opinion polls suggest that if a second referendum were held, it would be unlikely to yield a large majority in favor of leaving or remaining. As in 2016, when Brexit campaigners won narrowly with 52 percent of the vote, any new balloting is likely to produce a knife-edge result.

That means, say those who do not want the British back, that a Britain that decided to stay in the E.U. would have a powerful anti-Europe lobby that would be eager to take down any British leader who played a constructive or conciliatory role in European decision-making.

“It will be a new British prime minister coming to Brussels, explaining to us, ‘It’s a victory for us, it’s a victory for Britain and the E.U., but half of my people are very reluctant to continue to be members, so we need more opt-out provisions,’ ” or ways to avoid E.U. rules, Lamassoure said. The subsequent discussion would be likely to spur other countries to push for their own opt-outs, weakening the E.U. as a whole, he said.

A British reversal “has the potential of very seriously impeding decision-making,” said Fabian Zuleeg, the head of the European Policy Center, a Brussels-based think tank. “It isn’t a very attractive proposition anymore. The U.K. has burned so many bridges.”

Major decisions are looming, including on the mammoth seven-year E.U. budget that will determine the shape of E.U. priorities for years. It needs to be sorted out in the spring, and many policymakers in European capitals fear that Britain could use a continued membership inside the E.U. to hold them hostage and make tough new demands.

European trust in Johnson has quickly vanished. If he were toppled, his likeliest replacement would be Labour leader Jeremy Corbyn, a far-left figure who has long been skeptical of the E.U., because he views the bloc as favoring corporations over people. Many European diplomats doubt he would be significantly easier to work with than the Conservative Party leader.

Nor is there any guarantee that a future leader could not try, yet again, to pursue a departure from the E.U., creating more uncertainty and chaos.

“What sort of trust can you have in the U.K.?” said a senior European diplomat who is directly involved in Brexit negotiations, speaking on the condition of anonymity so as not to undercut what talks remain.

In the meantime, with E.U. companies struggling to plan for a jolting shift to their business model that could happen next month, or in three months, or not at all, their planning costs are mounting, further infuriating the Europeans.

European policymakers are coming to accept that it may be best to deal with the pain of Brexit and move on.

“The certainty of a deterioration can be better than continuous uncertainty without a new perspective,” Dutch Trade Minister Sigrid Kaag told Het Financieele Dagblad, a daily newspaper, on Monday.

That was the position of French President Emmanuel Macron at an emergency Brexit summit in April. Macron was an outlier then, pushing for a shorter Brexit extension than other leaders wanted to allow and arguing that British lawmakers should be forced to pick ibetween the existing withdrawal deal and a chaotic, no-deal Brexit. Now there is broader support for Macron’s point of view.

Still, some politicians in Europe remain open to a British reversal. In Eastern Europe, countries that fear Russian aggression and appreciate Britain’s robust military would prefer to keep it as close as possible. In Western Europe, some pro-E.U. leaders say any victory for the E.U., no matter how complicated, should still be embraced.

“More than ever, leaving the European Union, given geopolitical shifts and turmoil, is the worst that can happen,” said Norbert Röttgen, the head of the foreign affairs committee in the German parliament.

“We cannot afford to get upset with the Brits. This is a much too important country for the emergence of Europe as a constructive player in international relations,” he said.