LONDON — Prime Minister Boris Johnson offered Britons their first detailed glimpse of what a post-pandemic society might look like on Monday, announcing free twice-weekly coronavirus tests in England and Covid status certificates that would allow people with immunity into crowded nightclubs and sporting events.

The plans were the next step in the British government’s cautious reopening of the economy, and its first effort to tackle thorny questions about how to distinguish between people who are protected against the virus and those who are still vulnerable, as the country edges back toward normalcy.

“I will be going to the pub myself and cautiously but irreversibly raising a pint of beer to my lips,” Mr. Johnson said at a news conference at 10 Downing Street, as he listed the next round of relaxed restrictions.

Trying to strike a balance between public health and personal liberties, he said Britain would design a system to certify the Covid status of anyone seeking to enter higher-risk settings. While pubs and nonessential shops might be allowed to demand proof of Covid-free status, they will not be required to do so.

Britain has long resisted the idea of requiring people to carry identity documents, and for some in the country, this issue carries authoritarian overtones. The leader of the opposition Labour Party, Keir Starmer, recently suggested that Covid “passports” could be against the “British instinct.”

Mr. Johnson acknowledged the sensitivities and pointed out that the certification plan would not be rolled out for a few months. The government plans to test the program in pilot locations, from a comedy club and nightclub in Liverpool to the FA Cup soccer final at Wembley Stadium.

“You’ve got to be very careful in how you handle this,” he said, “and don’t start a system that is discriminatory.”

Starting next week, the prime minister said nonessential shops, hairdressers and beer gardens in pubs, in England would be allowed to reopen. But he was far more cautious about foreign travel, declining to say whether the government would stick to its earlier target of May 17 for lifting a ban on overseas vacations.

Britain plans to classify countries according to a traffic light system, with visitors from green countries not required to isolate themselves, visitors from amber countries required to isolate at home for several days, and those from red countries required to continue quarantining in hotels.

With more than 31 million people having gotten at least one vaccine jab, and the country still largely in lockdown, Britain has dramatically driven down its new cases, hospital admissions and deaths from the virus. As a result, Mr. Johnson’s focus has shifted to managing a steadily more open society.

Among his most ambitious plan is to offer free rapid testing kits to the entire population, so people can test themselves routinely. The kits, already used by hospitals and schools, will be available by mail or at pharmacies.

Public health experts applauded the gradual pace of government’s measures, which they said were appropriate for a country in which the virus was still circulating, even with declining death rates and a rapid vaccine rollout. But they expressed skepticism about the testing program, questioning whether people would have the incentive to put themselves through a test twice a week.

“Testing only works if people isolate, based on a positive result,” said Devi Sridhar, head of the global public health program at the University of Edinburgh. “But if they can’t go to work and will lose income, what’s the incentive to get tested?”

Britain’s experience with testing and tracing has been among the most abysmal parts of its pandemic performance. Even now, experts said, it only isolates between a quarter and a half of those who come into contact with people who test positive for the virus.

“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.”