Theresa May is set for a showdown with the hard Brexit wing of her party on Tuesday as the war over her withdrawal plan intensifies.

The Prime Minister will order her MPs to support a plan calling for the unpopular Irish backstop to be replaced with “alternative arrangements” to avoid a hard border on the island.

However, some Tories believe the proposal does not go far enough, with its architect, Conservative MP Sir Graham Brady, admitting the backstop could actually stay in the withdrawal agreement even if his plan was carried out.

Without the support of the most vociferously pro-Brexit MPs in her party, May could go down in yet another defeat this evening – exactly two weeks after she presided over the largest ever loss by a UK government in history.

The political horse-trading in Westminster came as the EU's deputy chief negotiator, Sabine Weyand, repeated Brussels' long-stated position that the withdrawal agreement would not be reopened – meaning the backstop could no be removed.

One Tory MP described the day's events to City A.M. as a "farce", while another said May is "literally on fantasy island on the backstop and the Brady amendment".

May told MPs she would be ordering them to vote in favour of Sir Graham's amendment during a packed meeting in Parliament on Monday evening.

One anti-Brexit MP described the move as a "capitulation", adding: "You think she can't back herself into a corner any further and it turns out she can."

However, just minutes before the PM revealed the government's support for Sir Graham's proposal, the European Research Group of Conservative Brexiters held its own meeting, culminating in pro-Brexit ringleader Jacob Rees-Mogg declaring his colleagues were in “no mood” to support the plan.

One member of Rees-Mogg's European Research Group of Tory MPs told City A.M. they believed the move to back Sir Graham's amendment had been long planned and was in effect an attempt to get Brexiters to support the government's plan without making any meaningful changes.

"There's a real sense of disbelief," they said.

While Sir Graham's amendment might not win the approval of MPs, a plan put forward to delay Brexit by up to nine months could pass if Jeremy Corbyn orders Labour MPs to back it on Tuesday.

While MPs were yet again caught up in trying to break the political deadlock, the British Retail Consortium warned a 'no deal' Brexit could lead to empty shelves in shops as 'just in time' delivery arrangements are disrupted.

The chief executives of Marks & Spencer, Waitrose, Sainsbury, Asda and Morrisons co-signed the letter which was sent to MPs, as did the heads of McDonalds, KFC, Co-op, Costcutter, and Lidl.