The new variant of coronavirus that has emerged in the UK may be associated with a higher mortality rate, Boris Johnson has warned.

The Prime Minister told a No10 press conference: “We’ve been informed today that 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.”

His warning came as the Government’s chief scientific adviser, Sir Patrick Vallance, said the variants which had emerged in South Africa and Brazil may be less susceptible to the vaccines that have been developed.

Sir Patrick said there was growing evidence from multiple sources that the vaccines will work against the UK coronavirus strain.

However, he said there was less certainty about the vaccines’ efficacy against those which had appeared in other countries.

“We are more concerned that they have certain features that they might be less susceptible to vaccines,” he said.

Sir Patrick said there were signs that there was an increased risk of death for those who have the new UK variant compared with the old virus.

He cautioned, however, that this was based on evidence which is “not yet strong”, and there was “no real evidence of an increase in mortality” among those in hospital with the variant.

“These data are currently uncertain and we don’t have a very good estimate of the precise nature or indeed whether it is an overall increase, but it looks like it is,” he said.

He said that for a man in their 60s, the average risk was that for a thousand people who got infected, roughly 10 would be expected to die – whereas with the new variant it might be 13 or 14.

The chief medical officer for England, Professor Chris Whitty, said there were signs coronavirus cases were falling – while hospitalisations in parts of England were beginning to “flatline”.

However, he said it will take weeks for death rate to start falling.

“The most recent seven-day rolling average is over 1,000 deaths a day,” he said.

“This is a very high rate and it will take longer to come down and will probably go up over the next week.”

Sir Patrick added: “The death rate is awful and it’s going to stay, I’m afraid, high for a little while before it starts coming down, that was always what was predicted from the shape of this.”