Theresa May is facing for a potentially crushing defeat for her Brexit deal in the House of Commons as last ditch appeals for support appear to have failed.

More than 100 Tory MPs have declared their opposition to May’s Withdrawal Agreement hammered out with Brussels, meaning the government could go down to one of the heaviest defeats of modern times.

The biggest post-war defeat was by 89 votes in 1979, according to academic Philip Cowley, of Queen Mary University London. 

But May, according to reports, is staring down a defeat of more than 200 votes, with the Telegraph calculating that the deficit could total 229 and Sky News estimating 225.

The Guardian reports the margin of defeat could be “more than 200” and the Mail cites “allies” of the PM acknowledging that “approaching 200″ will be the majority.

To give her a chance to go back to Brussels to get better terms, and then put a re-worked deal to a second vote of MPs, May is hoping for a defeat of less than 100.

The size of a defeat is crucial to what happens next as it will determine whether  May faces pressure to trigger a ‘Plan B’ immediately, face a vote of no confidence or any of a series of alternatives.

EXPLAINED: This Is How Crunch Brexit Vote Will Play Out – And What Happens If May Loses

The deal suffered its first official parliamentary defeat in the House of Lords on Monday night as peers voted by 321 votes to 152 – a majority of 169 – to reject it. The vote was indicative rather than binding.

On Monday evening, the Prime Minister insisted she was focused on winning the vote, telling Conservative rebels they risked handing the keys of No 10 to Labour leader Jeremy Corbyn.

Meanwhile, Corbyn indicated he was finally ready to table a vote of no confidence in the government if it loses in the Commons.

“Don’t be concerned, it’s coming soon,” he told a meeting of the Parliamentary Labour Party.

But with even rebel Tories likely to rally behind May if Labour attempts to trigger a general election, Corbyn’s aides have declared his ‘preferred’ or ‘default’ option was not a second Brexit referendum.

Downing Street has given little indication as to how the Prime Minister intends to proceed if she is defeated.

Under the terms of an amendment passed last week, she must table a motion on her ‘Plan B’ by Monday – although in practice she is unlikely to want to wait that long.

Some reports have suggested she could fly to Brussels – possibly as early as Wednesday – in an attempt to wring further concessions on the crucial issue of the Northern Ireland ‘backstop’.

But that could prove problematic if she has suffered a massive defeat, with EU leaders reluctant to give ground if they believe it is simply impossible for her to get the deal through the Commons.

Alternatively, she could to bring back the deal to the House for a second – or possibly even a third – time in an attempt to wear down the opposition.

HuffPost UK reported on Monday how cabinet ministers are ready to back plans to compel May to implement whatever ‘Plan B’ Brexit wins the backing of parliament.

Fresh moves to allow the House of Commons to seize control of the shape of the UK’s exit from the EU emerged on Monday, with former Tory ministers Nick Boles and Dominic Grieve both keen to give MPs the final say.

May will make her final appeal when she winds up five days of debate in the Commons before MPs head to the division lobbies.

Voting is due to begin at 7pm and could continue for around two hours, depending on how many amendments Speaker John Bercow calls before the final “meaningful vote” on the deal.

Those tabled include one by the Labour MP and chairman of the Commons Exiting the EU Committee Hilary Benn intended to block both May’s deal and a no-deal Brexit.