British Prime Minister Boris Johnson leaves 10 Downing Street for Prime Minister's Questions at the Houses of Parliament in London, Britain, Photo: Xinhua

The new UK coronavirus variant may be 30% more deadly, Boris Johnson said on Friday as he warned of stricter travel curbs and continued lockdown measures while the infection rate remains “forbiddingly high”.

In findings that dampened hopes of the increasingly prevalent B117 variant becoming less lethal over time, researchers on the government’s New and Emerging Respiratory Virus Threats Advisory Group (Nervtag) concluded that it may increase the death rate by 30%-40%.

Scientists urged tighter controls, and one said the news could take the country “back to square one”. It is also likely to increase the concerns of EU leaders, who were already considering strengthening border measures against the “British mutation”.

Addressing a Downing Street press conference on Friday, the prime minister struck an extremely cautious note when asked about the prospects of any imminent return to normal life, despite the rapid pace of the vaccine programme. One in 10 adults in England had been vaccinated, he said.

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 infe

“Currently, the rate of infection is forbiddingly high, and I think we have to be realistic about that,” he said. “I think we will have to live with coronavirus in one way or another for a long while to come. I think it is an open question as to when and in what way we can start to relax any of the measures. Obviously, we want to do everything we can to open up but only safely, only cautiously.”

Johnson hailed the speed of vaccine delivery, with 5.4m doses administered, putting the government on course for the 15 February deadline it set for immunising the first four priority groups. But he urged the public to continue to comply with restrictions.

The government’s chief scientific adviser, Patrick Vallance, shared early evidence about the UK variant, which first emerged in south-east England in September and has become prevalent across the UK. It was a key factor in the cancellation of most Christmas socialising, and has led to travel bans around the world.

Explaining the scale of the difference, he said for every 1,000 60-year-olds infected with the new strain, perhaps 13 or 14 might be expected to die, compared with 10 in 1,000 for the original strain.

The variant is also believed to be 30%-70% more transmissible. “There is no preferential age, it can affect anybody at any age, similarly to the original variant, the original virus,” Vallance said.

Dr David Strain, a senior clinical lecturer at the University of Exeter Medical School, said the possibility that the new UK variant was not only more contagious but also more deadly than previous variants was worrying.

“The key fact here is that all of the good work that’s been done with [the drug] dexamethasone and is being done with better treatment strategies has reduced the mortality in the UK by about a third. [If the new mortality figure is correct that] has now been lost and we are back to square one,” he said.

Gabriel Scally, visiting professor of public health at the University of Bristol, said: “We can’t go on having these variants coming at us like this, so we do need to do something stop them.”

The briefing heard there was good evidence of vaccines continuing to work against the variant.

But there were worrying early findings in relation to other variants identified in South Africa and Brazil. Vallance said they may have certain features making them less susceptible to vaccines, though there was no evidence they were more transmissible.

Earlier, footage emerged showing Matt Hancock, the health secretary, saying there was “evidence in the public domain” that the South African variant reduces vaccine efficacy “by about 50%”.

In a recording of an online webinar with travel agents this week, seen by MailOnline, Hancock said: “There is evidence in the public domain, although we are not sure of this data so I wouldn’t say this in public, but that the South African variant reduces by about 50% the vaccine efficacy.

“We’ve got some of the South African variant in Porton Down, and we’re testing it. We’ve got a clinical trial in South Africa to check that the AstraZeneca vaccine works. Nevertheless, if we vaccinated the population, and then you got in a new variant that evaded the vaccine, then we’d be back to square one.”

The news deepened a mood of pessimism in Downing Street about when lockdown restrictions could be lifted. Official figures released on Friday suggested infection rates were declining – but only very slowly.

The Office for National Statistics survey, based on swabs from randomly selected households, found that between 3 and 16 January, 1.9% of people in the community in England, or one in 55 people, had Covid – down from about one in 50 between 27 December and 2 January.

A further 40,261 people tested positive for the virus on Friday, and 1,401 deaths were reported. The weekly death figure, of 8,686, was up 16.4% on the previous seven days.

Chris Whitty, the chief medical officer for England, said there appeared to have been “a turning of the corner” in terms of infections but that cases were coming down from “an exceptionally high level”

“Overall, the number of infections is broadly going down, but it’s at a very very high level, and it’s extremely precarious,” said Whitty. “A very small change and it could start taking off again from an extremely high base.”

Vallance said: “The death rate’s awful, and it’s going to stay high for a little while before it starts coming down.”

Ministers are steeling themselves to tighten border measures in order to protect the hard-won gains of the vaccine programme, with some in government warning that tough controls could be in place for months.

Asked about tighter curbs, which ministers are expected to discuss on Monday, Johnson said: “We may need to go further to protect our borders.”

The government is preparing to launch a hard-hitting new advertising campaign featuring footage of severely ill Covid patients and exhausted medical staff, exhorting the public to stick to lockdown rules. The ads, which will run across TV, radio and social media, will carry the message: “Can you look them in the eyes and tell them you’re helping by staying at home?”