LONDON — Prime Minister Theresa May makes a last-ditch effort on Monday to convince rebel lawmakers to back her Brexit divorce deal, warning them that Britain’s exit from the EU is now in peril from politicians seeking to thwart it.

May will also set out new assurances from the EU that it does not aim to sever Northern Ireland from the rest of Britain under the deal’s most contentious term — a “backstop” requiring EU rules in the province until a better free trade plan emerges.

The fate of the United Kingdom’s March 29 exit from the EU is deeply uncertain as parliament is likely to vote down May’s deal on Tuesday evening, opening up outcomes ranging from a disorderly divorce to reversing Brexit altogether.

Facing the deepest crisis in British politics for at least half a century, May will use a speech in the leave-supporting city of Stoke-on-Trent in central England to say that lawmakers blocking Brexit is now a more likely outcome than leaving without a deal.

“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,” May was expected to say in a speech to factory workers in the leave-supporting city of Stoke-on-Trent in central England, according to advance extracts.

May warned lawmakers on Sunday that failing to deliver Brexit would be “catastrophic” for democracy, and her ministers said that thwarting the outcome of the 2016 referendum could lead to rise in far right populism.

As part of the effort to get the deal approved by the British parliament, the EU was due to set out some assurances in a choreographed exchange of letters on Monday, EU officials said. Brussels has repeatedly said however that the deal itself cannot be renegotiated.

The EU letter was expected to address the so called “backstop”, an insurance policy to prevent the return of border controls between the British province of Northern Ireland and the Irish Republic, an EU member.

“The point of that letter is to show we’re helpful, not punitive, it’s a communication exercise as probably nobody has read the withdrawal agreement — two pages are digestible for more people,” an EU official said.

“It is not a renegotiation. It’s just reconfirming and explaining what was agreed in December,” the official said.

Both opponents and many supporters of Brexit oppose the backstop for requiring Britain to obey EU rules indefinitely, long after it has given up say in shaping them.

The EU stresses that the backstop is not the EU’s preferred solution to avoiding a hard border, that it does not undermine the Good Friday Agreement, nor is it part of any covert attempt by the EU to “annex” Northern Ireland.

The Northern Irish party that props up May’s government said the EU’s letter did not go far enough.

“The letter isn’t legally binding,” Democratic Unionist Party deputy leader Nigel Dodds told BBC radio.

The UK was also expected to deliver its own letter to the European Council and Commission. May was scheduled to make a statement to parliament at about 1530 GMT, a government source said.

But with May’s deal facing opposition from all sides in the lower house of parliament, the House of Commons, the letters are unlikely to change the fundamental outcome of the vote.

“If we are brave, we have nothing to fear; and I fear the consequences of no Brexit far more than I fear no deal,” prominent Brexit campaigner Boris Johnson wrote in the Daily Telegraph. “We must have the courage to vote down this lamentable deal and kill it off once and for all.” — Reuters