By Zamira Rahim, CNN

The new Covid-19 variant first detected in the UK may be more lethal than previous strains, according to the country's Prime Minister Boris Johnson.

"We've been informed that in addition to spreading more quickly... there is some evidence that the new variant... may be more associated with a higher degree of mortality," Johnson said at a news conference on Friday.

"Both the vaccines we're currently using remain effective both against the old variant and this new variant," he added.

The UK's chief scientific adviser, Patrick Vallance, elaborated on the evidence.

Vallance said patients who were hospitalized with the new variant did not appear to have a higher risk of dying compared to those hospitalized with the original form of the virus.

"However, when data are looked at in terms of... anyone who has tested positive there is evidence that there is an increased risk for those who have the new variant compared to the old [one.]"

Vallance added that the evidence was not yet strong and that the data remained uncertain.

"If you took... a man in their 60s, the average risk is that for 1,000 people who got infected, roughly 10 would be expected to unfortunately die with the virus. With the new variant, for 1,000 people infected, roughly 13 or 14 people might be expected to die," he said Friday.

Vallance added: "That's the sort of change for that sort of age group, an increase from 10 to 13 or 14 out of 1,000 and you will see that across the different age groups as well, a similar sort of relative increase in the risk."

The new variant was first detected in southeast England and is believed to be between 30 to 70% more transmissible than the previous ones, Vallance said.

The UK recorded a further 1,401 coronavirus-related deaths over the past 24 hours, government data showed Friday -- up from Thursday's daily increase of 1,290 deaths.

The total number of people in the country who have died within 28 days of a positive test now stands at 95,981.

Vallance said Friday that there was "increasing evidence" that the existing Covid-19 vaccines would work against the UK strain but sounded a note of caution about the variants detected in Brazil and South Africa.

"We are more concerned that they have certain features which means they might be less susceptible to vaccines," he said, though he added that researchers needed "more clinical information" about the variants.

Chief Medical Officer for England Chris Whitty said Friday that there were "definite signs of improvement" in both the number of people infected with coronavirus in England and in hospital admission data.

The number of new Covid-19 infections registered in the UK over the last 24 hours is also up on Thursday's figure. A total of 40,261 additional cases have been confirmed, bringing the total number of cases recorded in the UK since the pandemic began to 3,583,907.

Public Health England has continued to call for citizens to adhere to government guidance, warning once again on Friday that "not everyone with coronavirus shows symptoms" and urging for people to stay at home.

The UK remains under a strict lockdown.