LONDON — Britain on Friday moved ahead with a 14-day mandatory quarantine on arriving travelers, and heavy fines for those who break the rules, outlining strict new travel restrictions as other European nations are thinking about easing theirs.
For weeks, the British government resisted the idea of a quarantine on travelers — even as the death toll from the coronavirus rose above that of neighboring nations — but on Friday, Priti Patel, the home secretary, brushed aside critics who think her plan is damaging, unworkable or simply too late.
At a news conference, Ms. Patel argued that when the virus was spreading freely at home, it made little sense to restrict travelers, but now that the pandemic was under control in Britain, the calculation had changed.
“It is to protect that hard-won progress and prevent a devastating resurgence in a second wave of the virus,” said Ms. Patel, explaining the timing of the measures. As travel increases after the lockdown, “imported cases could begin to pose a larger and increased threat,” she added.
Travelers whose final destination is in Britain will be required to give an address at which they must self-isolate and where they will be subject to spot checks by the police. Those who do not comply face a fine up to £1,000 ($1,218).
The plan, which is to start on June 8 and to be reviewed every three weeks, has several exemptions, including for truck drivers, seasonal farm workers, government officials and medical personnel, in addition to anyone arriving from Ireland, which has a common travel agreement with Britain.
One prominent traveler who could be exempt is Prime Minister Boris Johnson, who might meet face to face with other Group of 7 leaders, including President Trump, if a planned meeting is held next month in Washington.
Still, for Mr. Johnson, who has held out a vision of a “Global Britain” after the country’s departure from the European Union earlier this year, the enforcement of a quarantine on international travelers is another painful symbol of how the pandemic has upended his new government’s agenda.
Other Britons are holding on to more ordinary hopes, such as being allowed to travel for a late-summer vacation, which has added to tensions within Mr. Johnson’s Conservative Party over the merits of the quarantine plan. On Friday, the government said it was considering the idea of allowing “air bridges” to specific countries so that British tourists could head for Mediterranean beaches.
Medical experts said the decision to impose a quarantine was useful but overdue. It would have been more effective three months ago, when people arriving from Italy and other parts of Europe brought the virus into Britain.
“Of course, it is a valid move now, but what was right in February-March is also right in May-June,” said Bharat Pankhania, an expert in infectious diseases at the University of Exeter Medical School. “They needed to do it when the virus was at a much earlier stage.”
Dr. Pankhania said that Britain’s decision to sequester visitors from China early in the outbreak was valuable, but that the government’s decision not to do the same with European arrivals offset the benefits of that decision and contributed to a massive spread of the virus in the population.
“It strikes me as being very late and very hard to enforce,” said Anand Menon, professor of European politics and foreign affairs at King’s College London, who noted that in New Zealand quarantine took place in managed facilities.
“Here you have to give an address and the police might pop by,” he said. “But if you live at that address with five others, does the quarantine apply to them and, if not, what is the point?”
In handling the pandemic, Britain has often gone its own way, entering lockdown after many other nations had already done so and lifting such measures more slowly. Most countries restricted travel earlier and now have lower rates of infection than Britain, which has suffered more than 36,000 deaths.
Britain’s move to quarantine was so slow it is taking place as some other European nations are experimenting with relaxing travel curbs. Switzerland, Germany and Austria are allowing families divided by border closures to meet again, and there is a hope that the three countries will open their frontiers with one another, and with France, in the middle of June.
At home, the British government is trying to lift the lockdown but is taking a cautious approach, knowing that Britons remain nervous about the risk of a second spike in infections.
It is struggling to win support for its plan to reopen schools for some students on June 1, with the authorities in some English regions resisting and the Scottish government saying that it will keep schools closed until August.
Its task of persuasion was not helped by guidance published by a scientific committee showing that the government’s specific plan for reopening schools to some groups of children next month was not among the nine scenarios it had modeled.
One option the scientists seemed to prefer, which would have split classes between groups of students attending on alternate weeks, was not favored by the ministers.
There was a good deal of confusion regarding Ms. Patel’s quarantine plan. Initial reports, following a phone call earlier this month between Mr. Johnson and President Emmanuel Macron of France suggested that travelers from France would be exempt.
In a statement at the time, the government said, “No quarantine measures would apply to travelers coming from France at this stage” and that “any measures on either side would be taken in a concerted and reciprocal manner.”
But after political backlash in Mr. Johnson’s cabinet, that idea was scrapped.
Nick Thomas-Symonds, who speaks on home affairs issues for the opposition Labour Party, said he supported the plan but added that the government’s handling of arrivals into the country “lacked urgency, coherence and clarity from the outset.”
“If quarantine is needed, it should not have taken so long for measures to be introduced,” he said. “Too little thought has been given to testing and screening at airports.”
Airlines have warned of a potentially devastating impact on their operations, and a lobby organization, Airlines UK, expressed its concern earlier this month.
The chief executive of the low-cost carrier Ryanair, Michael O’Leary, has predicted that the quarantine would be impossible to police and described the plans for it as “idiotic.”