Vaccine certificates could be introduced in Britain as a way out of the coronavirus lockdown, opening up a path to normality.
But Prime Minister Boris Johnson on Thursday said there could be harmful side effects to a document that allows people with Covid protection access to restaurants or live music venues.
Some managers in the hospitality industry also have concerns.
Mr Johnson said there were “moral complexities” to consider as some people were unable to take a vaccine for health reasons.
He said it might only be possible to introduce the plan after all adults have been offered the vaccine, and that it would not be in place for April 12, when pubs have been scheduled to reopen with limits.
"There are three basic components: there's the vaccine, there's your immunity you might have had after you've had Covid, and there's testing," Mr Johnson said.
"They are three things that could work together.
"No decisions have been taken at all. One thing I will make clear is none of this is obviously going to apply on April 12, when it will all be outdoors anyway."
Mr Johnson spoke a few hours before the UK Parliament extended lockdown powers by six months.
"There are some people who, for medical reasons, can't get a vaccination," he said. "Pregnant women can't get a vaccination at the moment. You've got to be careful about how you do this.
"You might only be able to implement a thoroughgoing vaccination passport scheme, even if you wanted such a thing, in the context of when absolutely everybody had been offered a vaccine."
Patrick Dardis, chief executive of the Young's pub chain, said vaccine passports would be "unworkable".
Greene King, the City Pub Group and Shepherd Neame chains have also opposed the plan.
"It's absolutely fine to exclude people where there is a situation of bad behaviour or drunkenness, and that's already enshrined in law,” said Shepherd Neame chief executive Jonathan Neame.
"But if you're going to exclude people for what they are, or what they have not done, that's a wholly different issue that does touch on discrimination, civil liberties and in this case data protection issues."
The emergency powers bill passed by 484 votes to 76 with the backing of the opposition Labour party, although 40 Conservative MPs voted against it.
Throughout the pandemic, a libertarian caucus within the ruling Tory party has objected to restrictions to mitigate the spread of the virus, saying it led to loss of freedom and creeping authoritarianism.
Many also worry about the staggering economic cost of lockdowns.
As things stand, the UK's road map to normality is unaffected by the legislation, and emergency powers will only be enacted should the pandemic significantly worsen in the months up to September, when the extension expires.