Early evidence suggests the variant of coronavirus that emerged in the UK may be more deadly, Prime Minister Boris Johnson said.
The data has been assessed by scientists on the New and Emerging Respiratory Virus Threats Advisory Group, which has briefed government.
However, all the evidence remains at a preliminary stage.
Studies have already shown it can spread more easily than other version of the virus.
The new variant was first detected in Kent in September. It has since become the dominant version of the virus in England and Northern Ireland and has spread to more than 50 other countries.
Mr Johnson said: “In addition to spreading more quickly it also now appears that there is some evidence that the new variant, the variant that was first identified in London and the south east, may be associated with a higher degree of mortality.
“It’s largely the impact of this new variant that means the NHS is under such intense pressure.