The Delta variant of Covid-19 is now responsible for 99% of UK cases, figures showed on Friday.
Infections involving the strain first identified in India rocketed by 79% in a week with 33,630 new cases in just seven days, taking the total to 75,953.
But Boris Johnson, who on Monday ordered a four-week delay to easing restrictions, insisted the Government is likely to open up on July 19.
Speaking on a visit to Dewsbury, West Yorks, the Prime Minister said: “I’m very confident we’ll be able to go through with Step Four of the roadmap on the timetable that I’ve set out with treating July 19, as I’ve said, as a terminus date.
“I think that is certainly what the data continues to indicate.”
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England’s R rate, which reveals how quickly the virus is spreading, remained unchanged from last week at 1.2 to 1.4, meaning that the disease is growing exponentially.
Of the 75,953 Delta cases, 70,856 were in England, 4,659 in Scotland, 254 in Northern Ireland and 184 in Wales.
Another 10,476 infections were diagnosed in the past 24 hours, up a third in just seven days.
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Officials recorded another 11 deaths on Friday, taking the toll to 127,956.
But Covid-19 dropped to the 24th leading cause of death in England last month, Office for National Statistics figures showed.
A total of 333 deaths were due to the virus in May, the equivalent of 0.9% of all deaths registered in England.
The leading cause of death was ischaemic heart disease, followed by dementia and Alzheimer’s disease.
Patients with Delta are at more risk of being admitted to hospital than those with the Kent strain, which fuelled a winter spike in infections.
According to Public Health England’s latest variant technical briefing, as of Monday, 806 people had been hospitalised with the Delta strain, up 423 on the week before.
Some 527 patients were unvaccinated and only 84 of them had been given both doses.
There had been 73 deaths with the Delta variant in England within 28 days of a positive test. Of those, 34 were unvaccinated, 10 were more than 21 days after their first dose of vaccine and 26 were more than 14 days after their second dose.
UK Health Security Agency chief executive Dr Jenny Harries said: “It is encouraging to see hospitalisations and deaths are not rising at the same rate [as infections].
“But we will continue to monitor it closely.”