Early evidence suggests the variant of the coronavirus that emerged in the United Kingdom may be deadlier, Prime Minister Boris Johnson said.
However, there is still great uncertainty about the numbers – and whether vaccines are expected to work.
The data comes from mathematicians comparing the death rate in people infected with either new or old versions of the virus.
The new, more infectious variant has now spread widely across the UK.
Johnson told a Downing Street briefing: "As well as spreading more quickly, there now appears to be some evidence that the new variant - the variant that was first identified in London and the south-east - may be associated with a higher rate of high mortality.
"Mainly the impact of this new variant which means the NHS is under such intense pressure."
