The Delta variant now accounts for almost all of the UK's coronavirus cases, according to Public Health England.
Some 33,630 cases of the variant were recorded in the last week and PHE said "the most recent data show 99% of sequenced and genotyped cases across the country are the Delta variant".
Last week, the Delta variant made up 90% of COVID cases across the UK.
The Delta variant is thought to be 65% more transmissible than the Alpha (Kent) variant in households, according to PHE estimates.
Although previously dominant in the UK, the Alpha variant - blamed for the winter surge in cases - made up only 4,089 cases in the last week.
The risk of being admitted to hospital with the Delta variant of coronavirus is around double that of the Alpha variant, but two vaccine doses still provide strong protection against both.
Some 423 people have been admitted to hospital with the Delta variant since last week, taking the overall total to 806.
Of these, 527 were unvaccinated and only 84 of the 806 had received both doses - although it is unclear how long it had been since their second dose, as it takes a couple of weeks to build up protection.
It's also unclear how high the case fatality rate is - or the proportion of people with the virus who go on to die - as deaths usually occur weeks after infection and the majority of cases have been in more recent weeks.
The increase in Delta cases is largely among younger age groups, many of which are unvaccinated or have only just received their first dose.
Dr Jenny Harries, chief executive of the UK Health Security Agency, said it was "encouraging" that hospital admissions and deaths are not rising at the same rate as cases, but experts will "continue to monitor it closely".
Total cases of the Delta variant have been largely highest in areas of the North West, with Bolton recording a total of 4,684 confirmed or probable infections.
Cases of the variant have also been high in locations including Blackburn with Darwen, Birmingham, Bedford, Bury, Kirklees, Manchester, Stockport and Wigan.
About one in 520 people in the community in England are estimated to have had COVID-19 in the last week, up slightly from one in 560 the previous week, according to the Office for National Statistics.
Meanwhile in Wales, cases remain very low with one in 1,500 infected.
In Scotland, this figure was one in 600 and in Northern Ireland it was one in 610.