LONDON — Prime Minister Theresa May’s Brexit plans were in disarray on Tuesday as her government sought to plot a way around a ruling by the speaker of parliament that she must change her twice-defeated divorce deal to put it to a third vote.

After two-and-a-half years of negotiations, Britain’s departure from the European Union remains uncertain, with possible outcomes still ranging from a long postponement, leaving with May’s deal, a disruptive exit without a deal, or even another referendum.

In a move that added to the sense of crisis in London and exasperation in European capitals — on the eve of a crucial EU summit and days before the March 29 exit date — speaker John Bercow ruled that her Brexit deal had to be substantially different to be voted on again.

Brexit Secretary Steve Barclay said a vote this week on the deal was now less likely. But ministers were studying options, and he indicated the government still planned a third vote.

“This is a moment of crisis for our country,” he said.

May is due at an EU summit in Brussels on Thursday at which she will ask for a delay to Brexit as Britain seeks a smooth way to leave the EU after 46 years, with a transition period to soften the disruption to trade and regulations.

The EU’s most powerful leader, German Chancellor Angela Merkel, said: “I will fight until the last minute of the time to March 29 for an orderly exit. We haven’t got a lot of time for that.”

Her Foreign Minister Heiko Maas said: “If more time is needed, it’s always better to do another round than a no-deal Brexit.”

If it left this way, Britain would quit the EU’s 500 million-strong single market and customs union overnight, falling back on World Trade Organisation rules that could mean many import and export tariffs. It would face the prospect of manufacturing and financial market disruption, sharp economic contraction and border delays..

But France was blunter, saying a no-deal exit was possible.

“Grant an extension — what for? Time is not a solution, it’s a method,” said EU Affairs Minister Nathalie Loiseau. “If there is an objective and a strategy, it has to come from London.”

The 2016 referendum, which produced a 52-48 percent vote to leave, exposed deep divisions and has fuelled soul-searching about everything from secession and immigration to capitalism and British identity.

The crisis has left allies and investors puzzled by a country that for decades seemed a confident pillar of Western economic and political stability.

But Britons’ patience with negotiations may be running out. In a Comres survey in the Telegraph, nearly half of respondents said Britain would ultimately thrive if it left without a deal.

In addition to regulating the terms of departure, May’s deal promises to take Britain out of the EU single market and customs union, common fisheries and farm policies and the jurisdiction of the European Court of Justice at the end of a status-quo period in which new trade arrangements would be agreed.

The biggest issue is the so-called Northern Irish border backstop, an insurance policy aimed at avoiding post-Brexit controls on the United Kingdom’s border with EU-member Ireland.

Many Brexiteers and the DUP are demanding binding guarantees that the backstop will not trap the United Kingdom in the EU’s orbit indefinitely.

The Financial Times senior colleagues had told May she would have to set a timetable for her own departure in order to win over many rebels.

Barclay ruled out May asking Queen Elizabeth to cut short the entire parliamentary session, known as prorogation. — Reuters