Britain appears set for a full-blown constitutional crisis, with a cross-party group of senior lawmakers conspiring to sideline embattled Prime Minister Theresa May by seizing control of Brexit negotiations.

They want to reduce the power of the government to control legislative business in parliament, boosting the chances of lawmakers being able to table a series of motions to stop Britain from leaving the European Union without a deal, or even offering legislation for a second referendum on British membership in the EU that could lead to the country not leaving at all.

Ahead of Tuesday’s crucial vote in the House of Commons on her highly contentious Brexit Withdrawal Agreement, the result of two years of ill-tempered haggling with Brussels, May warned that if her draft deal with the EU is voted down, the most likely outcome would be Britain remaining an EU member.

Speaking to factory workers in Wales on Monday, May said if she loses the vote, “it is now my judgment that the more likely outcome is a paralysis in parliament that risks there being no Brexit.” She added, “There are some in Westminster who would wish to delay or even stop Brexit and who will use every device available to them to do so.”

Most observers — even ministers in her own divided cabinet — expect her to lose the vote on the withdrawal agreement, despite EU officials offering assurances Monday designed to diminish parliament’s opposition to the 585-page deal.

The big question is over the scale of the defeat, with some commentators predicting it could be the biggest rebuff a government has endured since 1924, when then-Labor Prime Minister Ramsay MacDonald lost a vote by 166, triggering the collapse of his government and a general election he lost.

Transition deal

With the deal, May has tried to square the circle between Britons who want to remain in the EU, or closely tied to it, and Brexiters.

The withdrawal agreement would see Britain locked in a customs union with the EU for several years while it negotiates a more permanent, but vaguely defined, free-trade settlement with its largest trading partner. In the temporary customs union, Britain would be unable to influence EU laws, regulations and product standards it would have to observe. It would not be able to implement free-trade deals with non-EU countries.

The transition was agreed to avoid customs checks on the border separating Northern Ireland and the Irish Republic, but British lawmakers fear Britain could be shackled indefinitely to the bloc, even if a final free-trade deal isn’t negotiated. Brexiters claim the Brexit agreement May negotiated turns Britain into a “vassal state, a rule-taker and not a rule-maker.”

On Monday, EU leaders sought to assuage lawmakers’ fears by saying it wouldn’t lock Britain in an indefinite transition. But opponents — both Brexiters and Remainers— appear unconvinced, calling assurances “meaningless,” as they don’t have the force of law, while the agreement, if passed by the British parliament, would.

Government vs. parliament

In readiness for May losing a key vote on the highly contentious Brexit Withdrawal Agreement in the House of Commons Tuesday, veteran lawmakers, including former ministers from her cabinet, are laying the groundwork to upend the centuries-old relationship between Britain’s government and parliament.

That relationship is built on No. 10 Downing Street being able to control the legislative agenda, with government business taking precedence over that of individual lawmakers or the opposition parties. The shift would set the stage for a constitutional showdown that could have significant consequences for how Britain is governed, as Brexit itself would, said analysts.

“We may now be witnessing one of the most fundamental shifts in (the) relationship between the government and parliament since William Lenthall, the speaker, defied King Charles I in 1642,” according to Philip Cowley, a political scientist at London’s Queen Mary University.

That defiance came on the eve of the English Civil War, when the king entered the Commons to arrest five rebellious lawmakers. Charles demanded to know where the lawmakers were, and Lenthall replied, “May it please your Majesty, I have neither eyes to see nor tongue to speak in this place but as the House is pleased to direct me, whose servant I am here.”

Role of John Bercow

While the current behind-the-scenes parliamentary maneuvering is unlikely to trigger a clash of arms, it has thrust the spotlight on current Speaker John Bercow, who when he was elected to the post in 2009, made no secret of his determination to strengthen the powers of parliament and lessen the dominance of the government.

Last week Bercow, a moderate Conservative but ardent Remainer, outraged No. 10 Downing Street and Brexiters by allowing an amendment to the government’s business motion for the vote on May’s Brexit deal.

The amendment was in breach of standard practice. Under standard parliamentary rules, government motions can’t be amended. But Bercow accepted it, announcing that if precedence was always followed, nothing would ever change.

Two groups of Brexit opponents are planning to use Bercow’s ruling last week to try to pull away from government control of the parliamentary timetable. One group wants to suspend the Brexit schedule, allowing them to postpone Britain’s earmarked EU departure date of March 29, and for more efforts to be made to shape a national consensus.

Other lawmakers are looking at ways to force a second referendum. On Sunday, one of May’s predecessors, former Conservative Prime Minister John Major, called for Britain’s exit from the EU to be delayed and control of future negotiations with the EU be controlled by parliament. He also called for another referendum.

“In the midst of chaos, it is always sensible to pause and think,” he said.

According to recent polls, a majority of Britons now want a second plebiscite. May has adamantly ruled out that option to break the parliamentary deadlock.

With this week shaping up to be one of the most tumultuous in the modern history of the House of Commons, Jeremy Corbyn, leader of Britain’s opposition Labor Party, has said that if May loses the vote on Tuesday, he will table a motion of no-confidence in the government.