The coronavirus variant first identified in India is responsible for more than half of new Covid-19 cases in the UK, Health Secretary Matt Hancock said on Thursday.
Mr Hancock said "more than half and potentially as many as three quarters" of new cases have been caused by the B1617.2 strain.
He said that in the past 24 hours, the UK recorded its highest number of new cases since April 12.
Public Health England data showed cases of the variant had risen by 3,535 to 6,959 since last week.
On Thursday, the UK reported 3,542 new cases and 10 deaths within 28 days of a positive Covid-19 test.
Over the past seven days, there was an increase of 20.5 per cent in new cases from the previous week.
“Now, as we set out our road map, we always expected cases to rise," Mr Hancock said. "We must remain vigilant.
"The aim, of course, is to break the link to hospitalisations and deaths so that cases alone no longer require stringent restrictions on people’s lives.
“The critical thing to watch is the link from the number of cases to how many people end up in hospital.
“The increase in cases remains focused in hotspots and we’re doing all we can to tackle this variant wherever it flares up.
"Over the past six months, we now have built a huge testing capacity at our disposal.”
Mr Hancock was joined at the regular briefing by the chief executive of the UK’s Health Security Agency, Dr Jenny Harries.
Dr Harries said a newly released report which found that the vaccines used in the UK had a “very, very strong” effectiveness against the Indian variant.
Hours earlier, Mr Hancock defended himself in the House of Commons, denying charges that he lied to the public and government officials several times throughout the pandemic.
He and Prime Minister Boris Johnson have been accused of mishandling the pandemic by the premier’s former aide, Dominic Cummings.
Mr Cummings told a select committee on Wednesday that Mr Johnson had overseen thousands of "unnecessary" deaths.
At the briefing, Mr Hancock defended the government’s handling of cases among care-home residents during the first wave of the pandemic, which Mr Cummings brought up before the committee.
The health secretary said that at the time, the government was dealing with an “unprecedented situation”, and he stressed the government had worked “incredibly hard to put in place what is needed to fight a pandemic”.
Mr Cummings told the committee that Mr Hancock "should have been fired for 15 or 20 things, including lying to everybody on multiple occasions in meeting after meeting" on issues such as testing in care homes and personal protective equipment supplies.