Boris Johnson faces defeat in the House of Commons over his plans to bring in vaccine passports after Labour indicated that it would oppose the measures.
A Labour spokesman said making people show proof of Covid jabs for "everyday access" to venues was "costly, open to fraud and impractical".
Mr Johnson said on Monday that he would change the law to require proof of two doses of a Covid vaccine for entry into nightclubs and "other venues where large crowds gather".
To do so he must hold a vote in the Commons and the Lords but is now facing opposition from both Labour and the Liberal Democrats, with defeat a real possibility.
At least 42 Conservative MPs have already signed a petition opposed to Covid certification being used for "general services, businesses or jobs". A rebellion on that scale could be enough to overturn Mr Johnson’s Commons majority of around 80 seats if all other opposition MPs voted against the measures.
The Labour spokesman said: "We need to see the detail of what the Government puts forward regarding vaccine passports. We oppose the use of Covid vaccination status for everyday access to venues and services. It’s costly, open to fraud and is impractical.
"Being double jabbed doesn’t prove you aren’t carrying the virus. Testing for access to venues would be more efficient, and would give people and businesses more certainty."
Mr Johnson said he would bring forward plans to require proof of two vaccine doses for clubs and other settings by the end of September. By then, everyone over 18 would have been offered two doses of a Covid vaccine under government plans, countering claims of discrimination for demanding checks.
The Prime Minister made it clear he was making the move in an attempt to drive up vaccine uptake among the young, with concern growing among Whitehall at signs of a drop-off in jabs.
Doubt remains over whether he will ultimately push ahead with the vaccine passports plan come September, given that he could drop the move if enough young people are jabbed. He has long been a critic of ID cards and initially decided against mandating vaccine passports only a fortnight ago.
Vaccine passports in large venues
The position of the SNP – which has the third most seats in the Commons – on vaccine passports remains unknown. Senior party figures indicated earlier in the year they could support the move when it emerged as a possibility, before later saying they would await proposals before deciding.
It is also unclear whether Sir Keir Starmer, the Labour leader who has tended to be more in favour of retaining Covid restrictions than the Government, could whip all his MPs to vote against the passports.
Meanwhile, lockdown-sceptic Tory MPs are threatening to boycott the Conservative Party conference in October in protest over the push to legislate on vaccine passports. One prominent rebel on Covid rules, who asked not to be named, questioned why club-goers would have to show proof of a vaccine by then, but not party conference attendees.
The MP told The Telegraph: "You can’t have one rule for the events enjoyed by the public and another for those attended by politicians. So I assume the Prime Minister will be insisting on vaccine passports for the Conservative Party conference which takes place in October.
"If he does, then I suspect significant numbers of Conservative MPs and activists will refuse to attend."