Early evidence suggests the variant of coronavirus that has emerged in the UK may be more deadly, Prime Minister Boris Johnson says.
However, there remains huge uncertainty around the numbers - and vaccines are still expected to work.
Studies have already shown it can spread more easily than other versions of the virus.
The new variant was first detected in Kent in the UK 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 told a media briefing in London: "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 [National Health Service] is under such intense pressure."
Public Health England, Imperial College London, the London School of Hygiene and Tropical Medicine and the University of Exeter have each been trying to assess how deadly the new variant is.
Their evidence has been assessed by scientists on the New and Emerging Respiratory Virus Threats Advisory Group (Nervtag).
The group concluded there was a "realistic possibility" that the virus had become more deadly, but this is far from certain.
Sir Patrick Vallance, the government's chief scientific advisor, said the evidence on lethality "is not yet strong".
He said: "I want to stress that there's a lot of uncertainty around these numbers and we need more work to get a precise handle on it, but it obviously is a concern that this has an increase in mortality as well as an increase in transmissibility."
The new variant appears to be around 30 percent more deadly.
For example, with 1000 60-year-olds infected with the old variant, 10 of them might be expected to die. But this rises to around 13 with the new variant.
This difference is found when looking at everyone testing positive for Covid, but analysing only hospital data has found no increase in the death rate. Hospital care has improved over the course of the pandemic as doctors get better at treating the disease.
The Pfizer and Oxford-AstraZeneca vaccines are both expected to work against the variant that emerged in the UK.
However, Sir Patrick said there was more concern about two other variants that had emerged in South Africa and Brazil.
He said: "They have certain features which means they might be less susceptible to vaccines.
"They are definitely of more concern than the one in the UK at the moment and we need to keep looking at it and studying this very carefully."
The prime minister said the government was prepared to take further action to protect the country's borders to prevent new variants from entering.
"I really don't rule it out, we may need to take further measures still," he said.
Last week the government extended a travel ban to South America, Portugal and many African countries amid concerns about new variants, while all international travellers must now test negative ahead of departure to the UK and go into quarantine on arrival.
Setback for hard-hit UK
The warning about the higher risk of death from the new variant came as a fresh blow after the country had earlier been buoyed by news the number of new Covid-19 infections was estimated to be shrinking by as much as 4 percent a day.
Data published earlier on Friday showed that 5.38 million people had been given their first dose of a vaccine, with 409,855 receiving it in the past 24 hours, a record high so far.
England and Scotland announced new restrictions on 4 January to stem a surge in the disease fuelled by the highly transmissible new variant of the coronavirus, which has led to record numbers of daily deaths and infections this month.
The latest estimates from the health ministry suggest that the number of new infections was shrinking by between 1 percent and 4 percent a day. Last week, it was thought cases were growing by much as 5 percent, and the turnaround gave hope that the spread of the virus was being curbed, although the ministry urged caution.
But the Office for National Statistics estimated that the prevalence overall remained high, with about one in 55 people having the virus.
Britain has recorded more than 3.5 million infections and nearly 96,000 deaths - the world's fifth-highest toll - while the economy has been hammered. Figures on Friday showed public debt at its highest level as a proportion of GDP since 1962, and retailers had their worst year on record.
- BBC / Reuters