A growing number of Tory MPs have revealed they plan to boycott the party’s annual autumn conference over fears they will need a vaccine passport to gain entry.

The outcry from Boris Johnson’s backbenchers is growing, after his surprise announcement on Monday that the documents will be required in some venues, including nightclubs, from the end of September.

MPs declaring they would not be attending include Steve Baker and Mark Jenkinson, who said on Thursday that he was double-jabbed and had booked a conference ticket but would refuse to go “if we’re excluding people on the basis of their vaccination status”. Lady Helena Morrissey also ruled herself out on the same grounds.

Plans for domestic certification had been shelved just a few weeks ago, though the government had admitted they could be brought back when the country would probably face a fresh surge of Covid and flu cases after the summer.

With infections already soaring, ministers have said the vaccine passports will be required in indoor, unventilated spaces where lots of strangers mix.

Downing Street has refused to provide a clearer definition of which venues will be affected, leaving open the possibility that they will include pubs and bars, prompting MPs to expect certificates at Tory conference in October in Manchester.

The Guardian revealed on Tuesday that some backbenchers said they would be skipping the gathering.

One said they “will not be going to any venue that requires this kind of thing, including my own party’s conference”.

They added that they were confident enough Tories will unite to kill off any attempts to make the vaccine passports mandatory, but if “for some inexplicable reason” the government does win a vote, they would boycott the event.

Another said that the “events of the weekend” – referring to Johnson and chancellor Rishi Sunak U-turning after initially refusing to self-isolate – had showed “very clearly that you can’t have one rule for politicians and another for the public”, meaning there was “no doubt” vaccine passports would be needed at Tory conference.

“As a result, it shouldn’t come as a surprise if a number of Conservative MPs and activists refuse to attend,” they said.

They were joined several days later by Steve Baker, deputy chair of the Covid Recovery Group of lockdown-sceptic MPs, who said that “with a heavy heart and apologies to event organisers”, he was minded to agree.

A Conservative party spokesperson did not rule out vaccine passports being mandatory for delegates, saying: “The party will be following government guidance.”