Senior Conservatives on Saturday urged Boris Johnson to ditch plans that would see many of England’s poorest pensioners paying more for their social care – or risk being forced by his own MPs into a humiliating U-turn.

The prime minister, still reeling from sleaze allegations and fury among “red wall” MPs over scaled-back rail investment in the north, is facing another potentially damaging Commons rebellion at the hands of an increasingly mutinous party.

The Observer has learned that several northern Tory MPs took part in an emergency call set up by care minister Gillian Keegan on Friday afternoon, during which she was said to have been “monstered” by backbenchers complaining that the plans were unfair and had not been fully explained or thought through.

According to MPs in on the call, former Tory chief whip Mark Harper challenged Keegan to produce more detailed analysis of the plans – which neither she nor two civil servants present was able to do. Harper then said it would not be good enough for her to produce details on the day of the vote, which is expected to be Monday or Tuesday.

Tory whips are understood to have been told by several senior Tory MPs that they are considering voting against the plans, or abstaining, unless they are amended to make sure pensioners would not be forced to sell their homes to pay for their care, as Johnson previously promised.

Jeremy Hunt, former health secretary and current chair of the health select committee, said it was “deeply disappointing” that the new plans were “not as progressive” as those put forward by Andrew Dilnot, the economist who drew up the original plans for a cap on individual contributions. He said it would now be up to government to improve entitlements once the cap had been introduced.

Damian Green, the former Tory cabinet minister, who was also on the call, told the Observer that the government should drop the plans and adopt a system that would guarantee that people could retain a percentage of their housing wealth.

“I would urge them to adopt a different approach,” Green said. “I think it would be infinitely preferable to guarantee that people can keep a percentage of their housing wealth rather than having a flat rate applying to the whole country.”

Tory WhatsApp groups were said to be full of comments from MPs – including many in red wall seats – talking about a potential rebellion unless the government backed down.

Last week, when MPs’ minds were focused more on troubles over sleaze and the decision to axe the eastern section of the high-speed rail line to Leeds, ministers announced changes to social care plans which would mean poorer pensioners would not, after all, be able to count means-tested payments by the state for their care towards a total cap of £86,000 for any individual. It is believed the change was made under pressure from the Treasury.

Critics said this meant that while someone who owned a £1m house would be able to protect more than 90% of their asset, someone with a home valued at £70,000, in a less wealthy part of the country, would lose almost everything.

Dr Dan Poulter MP, who works part time as a psychiatrist in the NHS, said that the unwelcome change to the plans was the result of government not having set aside enough money for social care when its main announcements on extra NHS funding and care reform was made in September.

“The initial set of proposals for a £86,000 lifetime cap of social care costs were strong and addressed the injustice of people having to sell their homes to pay for their care, but there were always questions about whether the government’s sums added up,” Poulter said.

“So, while this this policy change is surprising, I suspect it may well have been driven by the realisation that an extra £5.6bn, while welcome, was never going to be enough to meet both the care and workforce challenges in the social care system as well as to properly finance the introduction of an £86,000 cap on care costs. Unfortunately, it will be poorer pensioners who have relatively modest assets that will be most affected by these changes.”

When she announced the plans last Thursday, Keegan said they would “reduce complexity” and ensure that people “are not unfairly reaching the cap at an artificially faster rate than what they contribute”.

Analysis by the Observer shows that almost three-quarters of the seats the Tories won from Labour at the last election will be among those hit hardest by these changes.

Of the 54 seats the Conservatives won from Labour in 2019, 41 have average house prices below that level. In the Burnley constituency, for example, the average house is worth £99,950. In Darlington, it is £135,000 and in Durham North West it is £120,000, according to recent figures from the House of Commons library.

Appearing before a committee of MPs last week, Dilnot said that about 60% of older people who end up needing social care would lose out under the government’s plans.

“The people most harshly affected by this change are those with assets of exactly £106,000,” he said. “But everybody with assets of less than £186,000 would do less well under what the government is proposing than under the proposals we made and that were legislated for. That was a big change announced yesterday. It finds savings exclusively from the less well-off group.”

Shadow health secretary Jonathan Ashworth said: “Boris Johnson’s care plans are descending into chaos, with Tory MPs squabbling while ministers admit they haven’t even studied how the proposals disproportionately hammer those with modest assets.

“As civil servants confirm that these changes will clobber some of the poorest pensioners, many in the north and Midlands, Tory MPs must join with Labour in voting down this unfair care con and demand ministers come back with a fair alternative.”

Charles Tallack of the Health Foundation said the average house price in red wall seats of £160,000 meant they were “most likely to be affected by the proposed changes”. The type of people currently using care are also more likely to be worse off under the plans. The majority of people in care are women over 80, who have a median wealth of £156,000.