The early evidence suggests the new variant of coronavirus that emerged in the UK may be more deadly, country's 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, BBC reports.
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.
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."
Mentioning new UK variant could be up to 30% more deadly than original, he added, "For someone aged 60 the average risk is that for 1,000 people who got infected, roughly 10 would be expected to die with the old virus."
With the new virus, roughly 13 or 14 people might be expected to die, the UK PM said.
He said you would see this across the different age groups too.
He stresses there is uncertainty around these numbers but there seems to be an increase in mortality as well as transmissibility.