"There is evidence that there is an increased risk for people who have the new variant, compared to the old variant", said Sir Patrick Vallance, the government's Chief Scientific Adviser who joined Johnson at the briefing.
It was already classified as a more highly-transmissible variant but its risk level was so far not believed to be any greater than the original strain.
The fast-spreading "U.K. variant" of the coronavirus might also be more deadly, Prime Minister Boris Johnson said Friday, an alarming development that will burden the British health system and reverberate around the world.
"Out of 1,000 people aged in their 60s you would expect 10 [patients with COVID-19] to die".
When we look at severity and mortality, data for patients who are in hospital with the virus suggest the outcomes for those with the original variant look the same as the new variant.More news: Eli Lilly treatment cuts Covid-19 risk by 80% in nursing homes
Professor Lawrence Young, a virologist at Warwick Medical School, suggested the more serious concern was the evidence that the South African and Brazilian variants may be less susceptible to the vaccines.
While still in an early stage, the new evidence suggests the United Kingdom variant is roughly 30% more deadly. But he pointed out that people who have received the vaccine appear to be immune to the new variant, and people who have previously been infected with the original variant appear to be immune to the new variant.
British officials say they are confident that the vaccines that have been authorised for use against COVID-19 will be effective against the new strain identified in the country.
Mr Johnson said there could be no easing of the lockdown restrictions in England until it was clear the vaccination programme was working.
Concerns about newly identified variants have triggered a spate of new travel restrictions around the world.
Prof Horby defended the Government's decision to announce the news about the mortality rates from the new variant.More news: Remains of Troops Killed in New York Helicopter Crash Recovered
The Daily Express strikes a more optimistic tone over the variant, insisting the country can beat the "deadly new strain".
Britain has reported over 3.54 million cases of COVID-19, and some 94,500 deaths associated with complications with the respiratory disease.
Virus deaths have risen 16 per cent over the past week, while the number of people hospitalised with Covid-19 is approaching double the number seen during the worst days of the first wave of the pandemic in April.
Meanwhile, leaked documents have revealed plans to give everyone who tests positive £500 ($900) - but the Treasury said it not even heard of the proposal and livid allies of the PM said it wasn't going to happen.
"A very small change and it could start taking off again from an extremely high base", he said.More news: Tractor rally looms as talks between farmers and Modi govt fail