The UK recorded 7,393 new coronavirus cases on Thursday and another seven deaths.
On Wednesday, 7,540 coronavirus infections were recorded - the most since 26 February.
Fears are growing that the planned lockdown easing on June 21 will be delayed as new cases soar.
According to Professor Neil Ferguson, whose modelling was instrumental to the UK locking down in March 2020, it may be 60% more transmissible than the Alpha variant which swept across the UK over the winter peak.
Elsewhere, the World Health Organisation’s Europe director warned that the Delta variant first identified in India was “poised to take hold in the region” as many countries prepare to ease restrictions.
Dr Hans Kluge said it had shown signs of being able to evade some vaccines and that some vulnerable people remained unprotected.
“We have been here before,” he said. “Over the course of last summer, cases gradually rose in younger age groups and then moved into older age groups, contributing to a devastating resurgence.”
Dr Kluge said that the spike in Covid-19 cases ultimately led to more lockdowns and deaths in the autumn and winter of 2020, adding: “Let’s not make that mistake again.”
It comes as new figures suggest around six in every seven people in England aged 50 and over are now fully vaccinated against Covid.
An estimated 85.8% of people in this age group had received two doses of a vaccine by June 6.
The Government has set a target to offer both jabs to everyone aged 50 and over by June 21.
Within this age group, 91.8% of people aged 80 and over are now estimated to have received both doses, along with 97.4% of people aged 70 to 79 and 90.8% of people aged 60 to 69.
Some 72.5% of 50 to 59-year-olds are estimated to be fully vaccinated.
People in their 50s began to receive invitations for a first dose of vaccine in early March, with many likely to have been offered a second dose in recent weeks.
Additional reporting by the Press Association.