Researchers say that a highly infectious variant of the new coronavirus is 30 percent to 100 percent more deadly than earlier strong variants.
The variant was first found in Britain late last year and has since spread around the world.
The new study compared death rates among people in Britain infected with the new variant to those infected with other versions of the virus. Scientists said that the new variant's death rate was "significantly higher."
The variant is known as B.1.1.7.
The B.1.1.7 was first found in Britain in September 2020. It has since been found in more than 100 other countries. It has 23 mutations in its genetic material and some of those changes make it spread more easily.
Scientists say it is about 40 to 70 percent more infectious than earlier strong variants.
In the British study, infection with the new variant led to 227 deaths in a group of nearly 55,000 COVID-19 patients. That is in comparison with 141 deaths among the same number of patients infected with other variants.
The British variant also is able to spread quickly. "This makes B.1.1.7 a threat that should be taken seriously," said Robert Challen. He is a researcher at Exeter University who co-led the research.
The study was published in the British Medical Journal Wednesday.
Independent experts said this study's findings add to earlier evidence linking B.1.1.7 with a higher risk of death from COVID-19.
Lawrence Young is a virologist and professor of molecular oncology at Warwick University. He said the reasons for B.1.1.7's higher death rate are still not clear. But they "could be related to higher levels of virus replication" and that B.1.1.7 is more infectious.
Young warned that the British variant was likely the cause for a recent rise in infections across Europe.
I'm Alice Bryant.