Cases of the Delta variant in Britain increased by almost 80 per cent over the past week.
There were 33,630 new cases of the strain, which now accounts for almost all of the Covid-19 infections analysed by the country's genomic experts.
The rise of the Delta variant, first discovered in India, led UK Prime Minister Boris Johnson to announce on Monday that the final lifting of social restrictions would be delayed.
Britain hopes to use the four-week extension to vaccinate more of its population and keep hospital admissions down.
Scientists at Public Health England estimate that two doses of a Covid-19 vaccine offer 90 per cent protection against being admitted to hospital with the Delta variant.
Since the strain was first detected in Britain, 806 people have been taken to hospital, of whom most were unvaccinated and 90 per cent had not received both doses.
As of Friday, all adults in England can book their first dose of a vaccine. A target of offering everyone a first dose by July 31 was brought forward to July 19, the new date when Mr Johnson hopes to lift restrictions.
"Cases are rising rapidly across the country and the Delta variant is now dominant," said Jenny Harries, head of the UK's Health Security Agency.
“The increase is primarily in younger age groups, a large proportion of which were unvaccinated but are now being invited to receive the vaccine.
“It is encouraging to see that hospital admissions and deaths are not rising at the same rate but we will continue to monitor it closely.”
Delta dominates
In a weekly update, Public Health England said cases of the Delta variant climbed by 33,630 in the past week, taking the number of infections from the variant to 75,953.
On Thursday, Britain registered more than 11,000 new Covid infections in the biggest one-day increase since February.
The variant now accounts for 99 per cent of sequenced and genotyped cases.
The rise of the variant led to criticism of Mr Johnson's government for moving too slowly to restrict travel from India.
Covid has not thrown its last surprise at us
Chris Whitty
Public Health Englad said it was too early to tell whether Delta was more deadly than other variants, such as the Alpha strain, which caused Britain’s winter peak of infections.
But it is thought to lead to an increased risk of hospital admission compared with Alpha.
The government’s chief medical adviser, Chris Whitty, said on Thursday that Covid-19 had “not thrown its last surprise at us”.
He said there was likely to be a further surge of infections in the autumn and winter when respiratory illnesses typically flourished.
“We know that winter and autumn favour respiratory viruses and therefore it’d be very surprising if this particular highly transmissible respiratory virus was not also favoured,” he said.