A Cabinet minister has refused to deny that vaccine passports may be required to gain entry to the Conservative Party conference, in a move that has sparked talk of a boycott.

Environment Secretary George Eustice said there “could be a case” for attendees at the Tory conference in October having to prove they have been fully vaccinated against Covid-19.

As many as seven Tory parliamentarians, including former minister Steve Baker, have already signalled their intention to stay away from the Manchester event in protest at the mandatory jabs proposal.

Downing Street, however, defended the Prime Minister’s “right to mandate certification” after vaccines minister Nadhim Zahawi extended their scope beyond nightclubs.

We haven’t ruled that out but we are working through this

Mr Zahawi told the Commons on Thursday that vaccination entry conditions – which would not allow people to alternatively show proof of a negative test – could be applied to “very large structured events, such as business events, music and spectator sport events”.

Cabinet minister Mr Eustice said that remained the Government’s thinking, despite pressure from its backbenches.

“As I say, it could be that business conferences, including possibly party conferences where you have large numbers of people indoors, there could be a case for it there,” he told Sky News.

“We haven’t ruled that out but we are working through this.”

As well as Mr Baker, Workington MP Mark Jenkinson and Tory peer Baroness Morrisey have all said they would boycott a party conference requiring vaccine passports while Bolton West MP Chris Green also suggested he would.

Adding his name to the list, North West Leicestershire MP Andrew Bridgen told GB News: “I could never support compulsory vaccination – it is certainly coercion.

“I think it is unworkable.”

Political blog Guido Fawkes stated that Tory veteran Peter Bone and South Thanet MP Craig Mackinlay had also pledged to join a stand-off.

A boycott would be an embarrassment for Boris Johnson, who will hope he can get his agenda on track and unify the party after the pandemic forced 2020’s conference online and caused divisions among his ranks.

The Conservatives did not confirm that vaccine passports would be used at the conference, but did say: “The party will be following Government guidance.”

Reports have suggested that Boris Johnson, in the face of suggestions that his plan to make nightclubs accessible to only the double-jabbed in September is a ploy to encourage young people to take up the vaccine offer, will give MPs a vote on vaccine passports after the return from the summer recess.

The vote promises to be closely run, with Tory rebels planning to ally with Labour to defeat the proposal.

With at least 42 Tory MPs having signalled they would not vote in favour of vaccine passports, the Government faces the real prospect of a loss in the Commons.