The Prime Minister said the government will support an amendment set to be voted on tomorrow that calls for the controversial Irish border ‘backstop’ provision to replaced with “alternative measures”.

With the EU insisting that the withdrawal agreement cannot be changed, it sets Mrs May up for a decisive final clash with Brussels to avoid a no-deal Brexit.

But the Prime Minister bid to save her deal could be undermined by Brexiteers in her own party, who warned they would not vote for the backstop amendment, claiming it was too vague.

Tensions are rising over the prospect of a new Irish border. Picture: Charles McQuillan/Getty Images

MPs will vote tomorrow on backbench proposals for a Brexit ‘Plan B’, including an amendment supported by the chairman of powerful Tory 1922 Committee, Sir Graham Brady.

After the Prime Minister took questions at a meeting of Tory MPs on Monday evening, party chairman Brandon Lewis said a majority vote in favour of the Brady amendment would “give a very clear message” to Brussels that the backstop - which will keep the UK tied to EU regulations if a trade deal has not been agreed by the end of 2020 - must be changed.

Earlier, Downing Street said the Prime Minister was “absolutely committed” to leaving the EU with a deal on 29 March, but added: “Clearly if we are to obtain parliamentary support for that deal some changes are going to have to be made.”

Vote on Brexit ‘plan Bs’ could lead to deeper deadlock - Paris Gourtsoyannis

But European Commission spokesman Margaritis Schinas told reporters in Brussels that the Withdrawal Agreement was “endorsed by leaders and is not open for renegotiation”.

Commission vice-president Jyrki Kateinen said it would be “a stupid thing” for the EU to make concessions putting the remaining 27 members at a disadvantage simply to secure a deal.

Mr Kateinen said there was “no reason to give any concessions” to the UK and there was “not much room for manoeuvre” on the backstop.

Chief EU negotiator Michel Barnier’s deputy Sabine Weyand said it was “quite a challenge” to see how a majority in support of a deal could be constructed in Westminster, warning there was a high risk of the UK crashing out by accident.

Ms Weyand told a Brussels conference that negotiation on the Withdrawal Agreement was “finished”.

Dismissing calls for changes to the backstop as “like Groundhog Day”, she insisted the EU27 were unanimous in opposing any time limit.

Downing Street confirmed the Prime Minister will take her plan back to the Commons for a second “meaningful vote” as soon as possible after Tuesday’s debate, whether or not she secures more concessions from Brussels.

Following two more days of debate in the Commons, a second vote on the deal is likely to take place on 14 February - six weeks before the UK leaves the EU.

Mr Brady said his amendment would give Mrs May “enormous firepower” if it is passed.

The DUP - which props up the government - is still to reveal how it will vote on the amendment, but yesterday backed the Prime Minister in trying to “face down the stubbornness of Dublin and Brussels”.

There are also doubts over whether the Brady amendment will win the support of Tory Eurosceptic rebels in the European Research Group (ERG).

Senior Brexiteer Sir Bernard Jenkin said it was “deliberately vague because it’s meant to mean different things to different people”, while former Conservative leader Iain Duncan Smith was reported to have called on Mrs May to table her own amendment to provide greater “clarity and purpose”.

ERG chairman Jacob Rees-Mogg said the amendment was “just an expression of opinion” and warned there was “no appetite in ERG to support it as a stand-alone motion”.

In Monday’s meeting, Mrs May was cheered after being questioned by Boris Johnson, who asked how the Brady amendment would bring about meaningful change to the Brexit deal.

“She basically said, we won’t know unless you support us Boris,” Tory MP Simon Hart said.

Brexit: UK and EU on collision course over Irish border ahead of crucial vote