Like a speeding car whose brake lines have been cut, the coronavirus variant first spotted in Britain is spreading at an alarming rate and isn’t responding to established ways of slowing the pandemic, according to Danish scientists who have one of the world’s best views into the new, more contagious strain. Cases involving the variant are increasing 70 percent a week in Denmark, despite a strict lockdown, according to Denmark’s State Serum Institute, a government agency that tracks diseases and advises health policy.

“We’re losing some of the tools that we have to control the epidemic,” said Tyra Grove Krause, scientific director of the institute, which this past week began sequencing every positive coronavirus test to check for mutations. By contrast, the United States is sequencing 0.3 percent of cases, ranking it 43rd in the world and leaving it largely blind to the variant’s spread.

British Prime Minister Boris Johnson on Friday suggested for the first time that the variant may be more deadly than the original virus. Because it can spread more easily, it can also quickly overwhelm medical systems, turning previously survivable bouts with the virus into perilous ones if hospitals are full and medical care is limited.

Danish public health officials say that if it weren’t for their extensive monitoring, they would be feeling a false sense of confidence right now. Overall, new daily confirmed cases of the coronavirus in Denmark have been dropping for a month.

“Without this variant, we would be in really good shape,” said Camilla Holten Moller, the co-leader of the State Serum Institute group modeling the spread of the virus.

“If you just look at the reproduction number, you just wouldn’t see that it was in growth underneath at all,” she said.

But the British variant is spreading so quickly that Danish authorities project it will be the dominant strain of the virus in their country as early as mid-February.