The UK has reported 35,204 new cases of the Delta COVID variant in the latest week – a 46% increase.
Public Health England says two doses of vaccine are still effective at providing protection against risk of hospitalisation.
It takes the total number of confirmed cases to 111,157.
Last week, the Delta variant made up 99% of COVID cases across the UK.
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Some 42 of the latest cases are the Delta AY.1 sub lineage.
Delta plus is thought to be more transmissible than the original Delta variant, according to health officials in India.
The Delta variant, first identified in India, now comprises 95% of all sequenced cases, PHE added.
It said a new strain, Lambda, has been designated as a variant under investigation with six cases detected between 23 February and 7 June – of which five had been linked to overseas travel.
PHE said vaccines continue to have a “crucial effect on hospital admission and death”, adding there is currently no evidence that this new variant causes more severe disease or renders vaccines less effective.
The latest figures also showed a further 514 people were admitted to hospital in England with COVID-19 in the week up to 21 June. Of these, 304 were unvaccinated.
Dr Jenny Harries, chief executive of the UK Health Security Agency, said though the data suggests “we have begun to break the link between cases and hospitalisations”, she warned against complacency.
She urged people to get vaccinated and book their second jabs as soon as possible, adding: “Whilst vaccines provide excellent protection, they do not provide total protection, so it is still as important as ever that we continue to exercise caution.
“Protect yourself and the people around you by working from home where possible, and by practising ‘hands, face, space, fresh air’ at all times.”