LONDON — June 21 was supposed to mark England’s “freedom day,” when the country’s remaining coronavirus restrictions would end, but the government has opted to keep them in place at least through July 19 because of concern over rising cases and a fast-spreading variant.

Boris Johnson announced the delay late Monday. “I am confident we will not need more than four weeks and we will not go beyond July 19,” he said. “But now is the time to ease off the accelerator.”

Johnson had set the timeline for full reopening in February as British vaccination efforts gained steam. Local governments in Scotland, Northern Ireland and Wales set their own targets, but all had plans to relax rules in late June.

Britain has fully vaccinated 44.6 percent of its population, according to tracking by The Washington Post, compared with 43.4 percent of Americans who are fully vaccinated. But the spread in recent weeks of a coronavirus variant first identified in India has contributed to an alarming rise in new cases.

“We know the remorseless logic of exponential growth,” Johnson said, adding that the delay had been “a very difficult choice” but that he believed it would save thousands of lives.

British news outlets began reporting on the expected change Monday morning.

“Wait four it,” read the front page of the Daily Mirror on Monday, referring to the four-week delay, along with the words “Freedom day on hold.”

Appearing on Sky News on Monday, Edward Argar, a junior health and social care minister, confirmed government worries about rising cases of the delta virus variant, which was first discovered in India. He described it as 40 percent more infectious than the earlier alpha variant, first found in Britain.

“That’s what’s driving the spike. We’ve gone in a week from 12,000 cases to about 40,000 cases, and that’s a big jump,” he said, describing the variant as “highly, highly infectious.” Scientists estimate 90 percent of the new cases in Britain are from the delta variant.

The symptoms of the variant, known as the delta variant and labeled a “variant of concern” by the World Health Organization, include a headache and runny nose. The strain is now dominant in almost every region of the country, according to recent data.

The idea behind the delay, Johnson said Monday, is to get more people vaccinated, especially with their second dose, before all restrictions are lifted. Recent studies indicate that after a first dose of the Oxford-AstraZeneca or Pfizer-BioNTech coronavirus vaccine, people are only about 33 percent protected from the delta variant, but that increases to more than 80 percent with the second dose.

Britain has concentrated on getting as many people as possible a first dose of a coronavirus vaccine. About 41 million people, or nearly 79 percent of the adult population, have received at least one shot. Only 30 million people, however, are fully vaccinated.

Speaking to LBC Radio on Monday, Keir Starmer, leader of the opposition Labour Party, blamed what he called the government’s “pathetic” border policy for the delay in easing restrictions, arguing that if India had been added to the government’s travel red list sooner, the delta variant might not have spread as rapidly.

The British government has long been criticized for its handling of the pandemic, which has killed more than 128,000 people in the country.

The government has also been accused of not shutting borders or imposing travel restrictions soon enough — with many asking why and how the variant first identified in India was able to spread on British soil so freely.

Last week, U.S. officials said that the delta variant made up 6 percent of new cases in the United States, but that figure is growing, which has some experts worried.

The variant “could spike a new epidemic heading into the fall,” Scott Gottlieb, the former commissioner of the Food and Drug Administration, said on CBS’s “Face the Nation” on Sunday.

But Gottlieb said that he saw no reason to believe that vaccines would not continue to be effective.

That optimism was echoed in London on Monday, when Johnson and senior U.K. health officials emphasized the success of the vaccines against the variant. But they tempered that optimism with a dose of caution, saying that the virus was not going to disappear.

“We cannot simply eliminate covid,” Johnson said. “We must learn to live with it.”

Schemm reported from Dubai and Taylor reported from Washington. This report has been updated.

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