The new "mutant" variant of coronavirus spreading in the UK is more likely to kill people than the original version of the disease, Prime Minister Boris Johnson has revealed.
And Government scientists said they were concerned about variations of Covid-19 found in Brazil and South African - which might be resistant to vaccines.
Previously, it was thought that the new strain of Covid-19 spreading in the UK is easier to catch than the old version, but is no more likely to kill you once you have it.
However, Mr Johnson told a Downing Street press conference: "I must tell you this afternoon that we have been informed today that, in addition to spreading more quickly, it also appears today that there is some evidence the new variant may be associated with a higher degree of mortality."
The Prime Minister said the vaccination programme would continue - and one in ten adults have already received a first dose of a vaccine, including 71% of people aged over 80.
Sir Patrick Vallance, the Government's chief scientific adviser, said the mutant coronavirus had now become a "common variant", comprising "a significant number" of cases.
He said it spreads up to 70% more easily than the original virus. Like the original virus, it could affect anyone of any age.
Referring to the higher mortality rates, he said: "There is an increased risk for those who have the new variant compared to the old virus".
He said the evidence was "currently uncertain". However, he said that for every 1,000 people in their 60s who caught the old variant of Covid, roughly 10 would be expected to die - while for the new UK variant, 13 or 14 people would die.
Vaccines worked against the mutant virus spreading in the UK, he said.
But he also said there were other variants causing concern, one found in Brazil and one in South Africa. These variants of the virus "might be less susceptible to vaccines," although research was at an early stage, he said.
Sir Patrick said: "They are definitely of more concern than the one in the UK. We need to keep on looking at it and studying this very carefully."
Government data up to January 21 shows of the 5.8 million jabs given in the UK so far, nearly 5.4 million were first doses, marking a rise of 409,855 on the previous day’s figures and taking the seven-day rolling average to 306,880 doses per day.
The day-on-day increase saw the average number of first doses needed each day in order to meet the Government’s target of 15 million by February 15 fall for the first time, with the required daily rate now standing at 400,704.
A further 1,401 people had died within 28 days of testing positive for Covid-19 as of Friday, bringing the UK total to 95,981, while another 40,261 confirmed cases were reported.
It comes as data from the Office for National Statistics (ONS) infection survey shows there has been a slight drop in the number of people infected with coronavirus in England after lockdown measures were introduced.
The ONS estimated one in 55 people in private households in England had Covid-19 between January 10 and 16, down from one in 50 people for the Christmas period of December 27 to January 2.