The mutant coronavirus variant which emerged in the south of England may be more deadly than the original strain, scientists have warned.
The Government's chief scientific adviser Sir Patrick Vallance said that while there was still a "lot of uncertainty" around the data, it was a matter "of concern" that as well as spreading more quickly, the mutant strain could also lead to more deaths.
He also told a No 10 news conference that there was some evidence that coronavirus variants which had appeared in South Africa and Brazil may be less susceptible to approved Covid-19 vaccines than the original strains.
Boris Johnson warned that further measures could be required to stop the new variants entering the UK following the decision to suspend all the Government's travel corridors.
"We may need to go further to protect our borders. We don't want to put that (efforts to control the virus) at risk by having a new variant come back in," he said.
His warning came as England's chief medical officer Professor Chris Whitty said that while infections were falling and hospital admissions were beginning to "flatline" - the situation across the country remained "extremely precarious".
"A very small change and it could start taking off again from an extremely high base," he said. "If that happened again, we would be in really, really deep trouble."
Sir Patrick suggested the new variant could increase the mortality rate by nearly a third for men in their 60s who have Covid-19.
For a thousand people in that group who became infected with the old variant, roughly 10 would be expected to die - whereas with the new variant it might be 13 or 14, with similar increases in mortality rates across the age ranges.
"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 of concern that this has an increase in mortality as well as an increase in transmissibility, as it appears of today," he said.
However, Public Health England medical director Dr Yvonne Doyle has said that it is still not "absolutely clear" the new variant coronavirus which emerged in the UK is more deadly than the original strain.
Dr Doyle said more work was needed to determine whether that was actually the case.
"There are several investigations going on at the moment. It is not absolutely clear that that will be the case. It is too early to say," she told the BBC Radio 4 Today programme.
"There is some evidence, but it is very early evidence. It is small numbers of cases and it is far too early to say this will actually happen."
"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 it very carefully."
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