“There’s still no proper effort at supported isolation, and an obsession with testing rates with no apparent understanding of the purpose of testing,” said David King, a former chief scientific adviser to the British government who has been an outspoken critic of its response to the pandemic.

While Professor King credited the government with finally becoming more cautious, he said, “the level of the virus in the country is so high that there is no reason to think we are out of this yet.”

The announcement on Covid certification follows weeks of contradictory signals. In February, Nadhim Zahawi, the minister responsible for the vaccine rollout, described its use for anything other than foreign travel as “wrong and discriminatory.” Last month, Mr. Johnson suggested it might be up to individual pubs to decide whether to require Covid passports before serving customers.

Under the government’s current thinking, the certification would apply to people who are vaccinated, who recently tested negative for the virus, or who can prove natural immunity from having recovered from Covid.

Opposition comes both from defenders of civil liberties on the left and libertarians on the right. Last week, more than 70 lawmakers last week signed a letter opposing the “divisive and discriminatory use” of Covid passports. They included more than 40 Conservative lawmakers who are part of the Covid Recovery Group, a caucus of lawmakers that has criticized lockdown measures.

Writing in the Daily Telegraph, Graham Brady, who chairs an influential group of Conservative backbenchers, argued that Covid passports make little practical sense because many young people will probably not have been offered a vaccination by the time the government plans to reopen much of the economy. Fundamental principles were also at stake, he said.

“At the beginning of last year, patient confidentiality was a sacred principle and the idea that other people could inspect our medical records was anathema,” Mr. Brady wrote. “Now the state is contemplating making us divulge our Covid status as a condition of going to the pub or cinema.”

Given the skepticism of the Labour leader, Mr. Starmer, the government knows that if it goes too far, it could lose a vote on the measure in Parliament.

Still, some see the civil liberties arguments as more evenly balanced. Adam Wagner, a human rights lawyer and expert on Covid-related laws, said the government needed to tread carefully because of privacy issues and because “a system such as this could put them on collision course with anti-discrimination laws, for example for people who cannot get vaccinated because of a disability.”

But he added that there was nevertheless a valid civil liberties argument for introducing vaccine passports.

“Lockdown is a very serious imposition on everyone’s liberties and increasingly a hammer to crack a nut,” Mr. Wagner said. “One way to reduce the possibility of lockdown is to allow people who are not infectious, or are less likely to be infectious, to do more of the things that people normally do than those who are infectious or who are more likely to be infectious.”