LONDON (REUTERS) - The rapid spread of the Delta coronavirus variant has driven a 50 per cent rise in infections in England since May, a large prevalence study led by Imperial College London found on Thursday (June 17) after Prime Minister Boris Johnson delayed the end of restrictions.

The government said the data supported Mr Johnson's decision to push back the end of Covid-19 restrictions in England to July 19, citing the threat of the Delta variant first identified in India and the need to vaccinate more people.

The latest round of the React-1 prevalence survey, conducted between May 20 and June 7, found prevalence was 0.15 per cent, compared with 0.10 percent in the last set of data from late April to early last month.

"Prevalence is increasing exponentially, driven by younger ages... and it appears to be doubling every 11 days. Clearly, that is bad news," Dr Steven Riley, professor of infectious disease dynamics, Imperial College London, told reporters.

The study is one of the largest prevalence surveys in Britain, with 109,000 volunteers tested in its latest round.

Dr Riley added that Britain's high vaccination levels made it hard to predict how long that exponential growth would last, saying the rapid roll-out of vaccine doses to younger age groups should slow it down.

More than half the adult population of Britain have received two vaccine doses, and more than three-quarters of adults have received at least one vaccine dose.

Although the Delta variant is shown to substantially reduce the effectiveness of one dose of vaccine against mild infection, two doses still provide good protection against severe disease, early evidence has shown.

"These findings highlight the stark context in which we took the difficult decision to delay Step 4 of the roadmap out of lockdown," Health Minister Matt Hancock said, referring to the lifting of the last remaining restrictions in England.

"We all must hold our nerve that little bit longer as our vaccine roll-out continues."