Scientists say signs a new coronavirus variant is more deadly than the earlier version should not be a "game changer" in the UK's response to the pandemic.
Boris Johnson has said there is "some evidence" the variant may be associated with "a higher degree of mortality".
But the co-author of the study the PM was referring to said the variant's deadliness remained an "open question".
Another adviser said he was surprised Mr Johnson had shared the findings when the data was "not particularly strong".
A third top medic said it was "too early" to be "absolutely clear".
At a Downing Street coronavirus news conference on Friday, the prime minister 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."
Speaking alongside the PM, the government's chief scientific adviser Sir Patrick Vallance said there was "a lot of uncertainty around these numbers" but that early evidence suggested the variant could be about 30% more deadly.
For example, Sir Patrick said if 1,000 men in their 60s were infected with the old variant, roughly 10 of them would be expected to die - but this rises to about 13 with the new variant.
But one of the briefing's co-authors, Prof Graham Medley, told BBC Radio 4's Today programme: "The question about whether it is more dangerous in terms of mortality I think is still open."
"In terms of making the situation worse it is not a game changer. It is a very bad thing that is slightly worse," added Prof Medley, who is a professor of infectious disease modelling at the London School of Hygiene and Tropical Medicine.
A further 1,401 deaths in the UK within 28 days of a positive coronavirus test were reported on Friday, as well as 40,261 new cases.
Full article - Covid: 'More deadly' UK variant claim played down by scientists
Boris Johnson has said there is "some evidence" the variant may be associated with "a higher degree of mortality".
But the co-author of the study the PM was referring to said the variant's deadliness remained an "open question".
Another adviser said he was surprised Mr Johnson had shared the findings when the data was "not particularly strong".
A third top medic said it was "too early" to be "absolutely clear".
At a Downing Street coronavirus news conference on Friday, the prime minister 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."
Speaking alongside the PM, the government's chief scientific adviser Sir Patrick Vallance said there was "a lot of uncertainty around these numbers" but that early evidence suggested the variant could be about 30% more deadly.
For example, Sir Patrick said if 1,000 men in their 60s were infected with the old variant, roughly 10 of them would be expected to die - but this rises to about 13 with the new variant.
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But one of the briefing's co-authors, Prof Graham Medley, told BBC Radio 4's Today programme: "The question about whether it is more dangerous in terms of mortality I think is still open."
"In terms of making the situation worse it is not a game changer. It is a very bad thing that is slightly worse," added Prof Medley, who is a professor of infectious disease modelling at the London School of Hygiene and Tropical Medicine.
A further 1,401 deaths in the UK within 28 days of a positive coronavirus test were reported on Friday, as well as 40,261 new cases.
Full article - Covid: 'More deadly' UK variant claim played down by scientists