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.
The announcement followed a briefing by scientists on the government's New and Emerging Respiratory Virus Threats Advisory Group (Nervtag) which concluded there was a "realistic possibility" that the variant was associated with an increased risk of death.
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.
Another 1,348 deaths within 28 days of a positive coronavirus test were reported in the UK on Saturday, in addition to 33,552 new infections, according to the government's coronavirus dashboard.
There is huge uncertainty in the evidence on how lethal the variant is.
The scientific experts that reviewed the data used a precise phrase saying it was a "realistic possibility" the new variant is more deadly.
That means there's a roughly 50-50 chance it will turn out to be true.
With time, and sadly more deaths, the picture will become clearer.
While people debate the uncertainties though, we already know this variant has the ability to kill more people than the old ones.
A virus that spreads faster (this one is 30-70% faster) will infect more people, more quickly, putting a greater strain on hospitals and leading to a sharper spike in deaths.
It is why viruses becoming more transmissible can be a bigger problem than ones becoming more deadly.
Nervtag's chairman Prof Peter Horby defended the government's "transparency" in making the announcement.
"Scientists are looking at the possibility that there is increased severity... and after a week of looking at the data we came to the conclusion that it was a realistic possibility," he said.
"We need to be transparent about that. If we were not telling people about this we would be accused of covering it up."
But Dr Mike Tildesley, a member of Sage subgroup the Scientific Pandemic Influenza Group on Modelling (Spi-M), agreed it was too early to draw "strong conclusions" as the suggested increased mortality rates were based on "a relatively small amount of data".
He told BBC Breakfast he was "actually quite surprised" Mr Johnson had made the early findings public rather than monitoring the data "for a week or two more".
"I just worry that where we report things pre-emptively where the data are not really particularly strong," Dr Tildesley added.
Public Health England medical director Dr Yvonne Doyle also said it was not "absolutely clear" the new variant was more deadly than the original.
"There is some evidence, but it is very early evidence. It is small numbers of cases and it is far too early to say," she told the Today programme.
Meanwhile, senior doctors are calling on England's chief medical officer to cut the gap between the first and second doses of the Pfizer-BioNTech Covid-19 vaccine.
The British Medical Association told Prof Chris Whitty an extension to the maximum gap between jab from three weeks to 12 weeks, to get the first dose to more people, was "difficult to justify".