"In addition to spreading more quickly, it also now appears that there is some evidence that the new variant identified in London and the southeast may be associated with a higher degree of mortality," he said at a press conference.

Patrick Vallance, the government's chief scientific adviser, cautioned that the new findings were based on a preliminary assessment of early data.

Among those who tested positive for COVID-19, there is "evidence that there is an increased risk" of death for those with the new variant compared with the old virus, Vallance said at the same press conference.

"These data are currently uncertain and we don't have a very good estimate of the precise nature or indeed whether it is an overall increase but it looks like it is," he said.

The UK's mutant strain emerged late last year and spread quickly in London and the southeastern English county of Kent, putting a further strain on hospitals.

It also raised alarm across the globe, with countries imposing fresh travel restrictions in an attempt to keep it at bay.

There is widespread consensus that the strain is more transmissible but until now there had been no public suggestion from high-level UK officials that the strain was deadlier.

Australian Associated Press