Theresa May will make one final appeal to the Commons to pass her Brexit deal, amid speculation among many Conservative MPs that the expected defeat could spark a cross-party plan or moves to oust the prime minister.

May is to start her most crucial week as prime minister with a speech at a factory in Stoke-on-Trent, where more than two-thirds of people voted to leave the EU. Reiterating her recent pleas, May will tell MPs that voting down her proposals on Tuesday would destroy faith in politics, and could mean that Brexit does not happen.

“I ask MPs to consider the consequences of their actions on the faith of the British people in our democracy,” she is due to say.

“What if we found ourselves in a situation where parliament tried to take the UK out of the EU in opposition to a remain vote? People’s faith in the democratic process and their politicians would suffer catastrophic harm. We all have a duty to implement the result of the referendum.

“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 will say in her speech.

But with less than 48 hours to go before the vote, there were few signs of the shift in opinion May hoped for when she postponed the vote before Christmas. A series of Conservative MPs have said that, while the margin of defeat remained unclear, a loss appeared inevitable.

“I don’t think the defeat will go into three figures – I’d expect between 50 and 100,” said one pro-Brexit MP who strongly supports May’s plan. “It depends in part how many Labour MPs are willing to vote for the deal.”

It is understood three Labour MPs, Caroline Flint, Gareth Snell and Lisa Nandy, who ministers had hoped would support the plan after the government adopted their amendment on workers’ rights, will vote against it.

Conservative backbencher Andrew Mitchell said nothing that had happened in recent days to change his mind, and the former chief whip said he was going to vote against May’s deal. “I think it creates more problems than it solves,” he said.

A heavy defeat could jeopardise May’s position just a month after she won a no-confidence vote. One usually loyal MP said: “If the loss is large there will be some pressure on her to go. There is a sense she has reached the end of the road.”

Another well-connected Tory MP said: “In parliament, the prime minister is basically there as the person who can deliver votes most of the time. If you can no longer do that on a sustained basis, it makes a mockery of the post. That doesn’t mean that she can’t carry on. What it does mean is the job becomes almost ceremonial.”

MPs and cabinet sources said that, if May loses, she would be expected to make a statement to the nation saying that she will seek new concessions from Brussels before putting her revised plan back to parliament.

While there is much speculation that she could try a different tack, such as seeking cross-party consensus for a softer Brexit, this has not been publicly acknowledged.

On Sunday, Brexit secretary Stephen Barclay and transport secretary Chris Grayling both declined to discuss an alternative. “Let’s cross that bridge if and when it happens,” Grayling told Sky News. “Right now her focus, my focus, the government’s focus is on convincing people that actually this is the best thing to do.”

Cabinet sources said that while some ministers, such as Amber Rudd and David Gauke, had urged May to consider a backup approach, little concrete work had been done. One source said: “The view is that there’s still a little bit of steam left in the PM’s deal. If someone is coming up with a brand new plan B, like Norway, then they’re doing it very quietly in a dark room.”

MPs will next week attempt again to seize control of the process, with groups pushing for a Norway-style soft Brexit and a second referendum.

One Conservative MP said the Norway plan – to keep the UK inside the EU’s single market – was gaining traction: “It is the only plan B with a measure of cross-party support. It would get you 80 or 90 Labour votes.”

Separately, a cross-party group of MPs from the Lib Dems, Labour, Conservatives and SNP will on Monday publish two draft bills designed to pave the way for a new referendum. One of these would give voters the option of voting for the government’s deal or staying in the EU.

Among those behind the plan is the Tory former attorney general Dominic Grieve, who was accused by some newspapers on Sunday of leading a coup against Brexit in alliance with the Commons Speaker, John Bercow.

Grieve, whose amendment obliging May to respond to the defeat of her plan within three days was passed by MPs last week, told the Guardian the reports were “rubbish”, and had seen him receive death threats.

“I can emphatically say it is totally untrue that I am in a conspiracy with the Speaker either to stop Brexit or to change the standing orders of the house,” he said.