Boris Johnson faces the biggest rebellion of his premiership on Tuesday over new Covid restrictions amid warnings he has galvanised previously loyal MPs against him.

Critics have warned that up to 100 MPs could oppose plan B changes across a series of Commons votes, forcing the government to rely heavily on Labour for them to pass.

Dozens of backbenchers – and some junior ministerial aides – have expressed opposition to requiring people attending large events to prove their vaccine status or a recent negative test result. Some are poised to rebel or abstain despite never having voted against restrictions before. 

While a backlash by some cabinet ministers led to the plans for vaccine passports being watered down so that proof of a negative test result would be allowed, too, that has not done enough to assuage concerns voiced by Steve Baker, deputy chair of the Covid Recovery Group, that the UK is drifting towards authoritarianism.

The documents will be required for indoor venues of more than 500 people or outdoor settings of 10,000 people. Keir Starmer confirmed on Sunday that Labour would support all plan B measures.

However several backbenchers who have never voted against Covid measures told the Guardian they would probably do so. One said the prime minister’s behaviour after recent scandals had “galvanised more of us to no longer blindly follow the orders cooked up by whips”.

A frontbencher also said: “If they go overboard with the whipping on this, it’s going to hasten Boris’ departure – and it’s already been hastened quite a lot.”

Simon Fell, a member of the 2019 intake who has not voted against Covid restrictions before, said while he had “no doubt about just how serious this new variant could be”, he would not support Covid passports. He said: “We are not a ‘papers please’ society, and such passports would be an unethical and discriminatory step. I will not be supporting their introduction.”

Tom Randall and Alicia Kearns, both new Tory MPs and members of the 1922 Committee executive, are set to stage their first rebellion over Covid restrictions. Randall said “the case for certification had not been made”. And Kearns said Covid passports were “being proposed before we receive a detailed assessment of the severity of the Omicron variant”, adding: “I cannot justify introduction of Covid-19 passports just in case and without the data, and given that evidence from other countries is that vaccine passports don’t work.”

Among those sceptical of the Covid passports plan are about six parliamentary private secretaries (PPSs) – MPs who act as aides to ministers and are expected to vote in line with the government. Jacob Young, a PPS to Michael Gove, is believed to be among them.

In a bid to curb the number who would be forced to resign if they abstain or vote against the measure, Sajid Javid, the health secretary, had a meeting with some PPSs on Saturday.

William Wragg, who sits on the executive of the 1922 Committee of backbench Tory MPs and heckled Javid in the Commons chamber last week, urging him to resign, said he felt “immense frustration that the government should be pursuing this policy, which on previous occasions they tried to reassure us that they wouldn’t be doing”.

Wragg said plan B was “jumping the gun” and “going too fast”, taking aim at any attempt to “coerce” people to get vaccinated by introducing Covid passports.

A frustrated minister predicted a “big rebellion” and said Johnson would probably be “wounded” by it for having to rely on Labour votes to pass such an important motion.

The minister told the Guardian that many Tories would not vote for Covid passports because Labour’s backing meant there was no chance the government would be defeated, but hit out at the “righteous indignity” of some colleagues. The minister added Johnson still seemed “clueless” on how to tackle Covid through the winter and there was a “genuine level of uncertainty” about whether more restrictions would follow, but insisted: “A proportionate, precautionary response is not unreasonable. This doesn’t pass the test of people’s liberty being fatally withdrawn.”

The previous biggest vote where Conservative MPs voted against the government was over changes to the tier system in December 2020, with 55 opposing the measures and 16 abstaining. Other rebellions have included more than 60 Tories opposing the 10pm curfew for hospitality venues enforced last autumn and pushing back the final stage of the reopening roadmap in June 2021.

Nikki da Costa, a former senior adviser in No 10, noted: “Whips’ rule of thumb is rebellion breeds rebellion – it’s easier ‘next time’.”

Nadhim Zahawi, the education secretary, tried to reassure people the Covid passports would only be temporary, as they have a sunset clause and will be reviewed on 5 January. He said it was only for “high risk events” that either a lateral flow test or vaccine status certificate would be required.

“That is, I think, proportionate as we see where we hopefully arrive at with the booster, because the boosters are to protect the most vulnerable,” Zahawi told the BBC’s Andrew Marr show. By Aubrey Allegretti Political correspondent, Yahoo News