Video report by ITV News Political Correspondent Daniel Hewitt
The prime minister told a Downing Street press conference: "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."
He said the NHS is under "intense pressure" due largely to the impact of the new variant, but reassured that both vaccines being used in the UK - Pfizer and AstraZeneca - "remain effective" against both the older strain and the new one.
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Mr Johnson added: "It's more important than ever that we all remain vigilant in following the rules and that we stay at home, protect the NHS and thereby save lives."
"All current evidence continues to show that both the vaccines we're currently using remain effective both against the old variant and this new variant."
There are concerns the South African variant of coronavirus reduces effectiveness of vaccines "by about 50%", according to what Matt Hancock said in a leaked recording obtained by MailOnline.
Asked about the health secretary's comments, Mr Johnson said: "We may need to go further to protect our borders.
"We don't want to put that [all the effort to control the virus] at risk by having a new variant come back in."
Mr Johnson revealed 5.4 million people across the UK have now received their first dose of Covid-19 vaccine and a record 400,000 vaccinations were carried out in the past 24 hours.
Sir Patrick Vallance said evidence is growing from multiple sources that vaccines will work against the UK coronavirus strain, but variants identified in South Africa and Brazil are of more concern.
The chief scientific advisor told the press conference: "There's increasing evidence from laboratory studies that the variant in the UK will be susceptible to the vaccines.
"There's increasing confidence coupled with a very important clinical observation that individuals who have been infected previously and have generated antibodies appear to be equally protected against original virus and new variant."
But Brazilian and South African coronavirus variants "might be less susceptible to vaccines".
"We know less about how much more transmissible they are. We are more concerned that they have certain features that they might be less susceptible to vaccines.
"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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On the impact of the new variant, which first emerged in Kent, Sir Patrick echoed the PM, saying it appears to come with "an increase in mortality as well as an increase in transmissibility".
He said out of a thousand 60 year olds, on average 10 would be expected to die if infected with the older variant.
With the new variant, for a thousand people infected, roughly 13 or 14 people might be expected to die.
It was earlier revealed a further 1,401 deaths had been recorded as of Friday, bringing the UK's total coronavirus death toll to 95,981.
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England's chief medical officer Professor Chris Whitty said it will take weeks to see significant drops in the number of people in hospital, with the current seven-day rolling average at more than 1,000 deaths a day.
"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 said the UK's current death rate is "awful" and it is "going to stay high, I'm afraid, for a little while before it starts coming down".
"I think the information about the new variant doesn't change that," he added.
But he said delaying the second dose of the vaccines does not create a substantial risk of making the virus more dangerous as it mutates to survive improved immunity.
He said: “The most risky thing in terms of new mutations is to have very high prevalence.
“The more the virus is replicating and transmitting between people, the more likely the chance that it will get a mutation, and alter, and that’s what’s happening around the world, and that’s why we’re seeing the same mutations pop up everywhere.
“These mutations have not come about because of vaccine pressure or anything else, they seem to be mutations that the virus accumulates naturally during replication as it wants to get more efficient at transmitting.
“So, I think that’s the biggest risk. There’s always some risk if you start to have partial immunity, but there’s also a benefit, which is that partial immunity can actually stop the infection quicker.”
It comes after Robert Peston reported the government's New and Emerging Respiratory Virus Threats Advisory Group (Nervtag) had concluded that the new strain "may be a bit more lethal than the original strain".
Nervtag member, Professor Neil Ferguson said: "It is a realistic possibility that the new UK variant increases the risk of death, but there is considerable remaining uncertainty.
"Four groups - Imperial, LSHTM, PHE and Exeter - have looked at the relationship between people testing positive for the variant vs old strains and the risk of death - that suggests a 1.3-fold increased risk of death.
"So for 60 year-olds, 13 in 1000 might die compared with 10 in 1000 for old strains.
"The big caveat is that we only know which strain people were infected with for about 8% of deaths."
Shadow health secretary Jonathan Ashworth, commenting on the news that the new UK Covid variant is potentially more deadly, said: "This is deeply alarming news, not least because Boris Johnson assured the nation back in December there was no evidence the variant was more dangerous.
“We urge ministers to go further and faster on vaccination roll-out now to save lives, and introduce proper financial support for those needing to isolate to help breaks chains of transmission.”