By Alistair Smout and Sarah Young
OXFORD, England (Reuters) -The prevalence of COVID-19 infections in England almost doubled in the last week of May and the estimated reproduction "R" number crept up as the "Delta" variant became more widespread, raising worries about the country's unlocking plans.
COVID-19 restrictions in England are due to end on June 21, but the swift spread of the Delta variant first detected in India is now threatening the timetable.
British Prime Minister Boris Johnson said on Wednesday he would be cautious in lifting restrictions but there was nothing in the current data to suggest a delay.
But on Friday, the Office for National Statistics said an estimated 1 in 640 people in England had COVID-19 in the week ending May 29, compared to 1 in 1,120 a week earlier, marking the highest proportion since the first half of April.
Britain's health ministry estimated that the reproduction "R" number in England was 1.0 to 1.2 - the second week it has been over 1 - and the epidemic could be growing by as much as 3% each day.
Asked if the data was moving in the wrong direction, Health Secretary Matt Hancock said the rise in cases was not unexpected given the easing of restrictions in recent months.
"The critical thing is the impact on the number of people who end up in hospital for any given number of cases," he told Reuters after a meeting of G7 ministers in Oxford, central England, on Friday.
"That link has been broken by the vaccine, but it hasn't been completely severed yet, and that's one of the things that we're watching very carefully."
He said that until data had been assessed, it was too early to say what the decision would be on final easing of lockdown.
Britain as a whole reported 6,238 new coronavirus infections on Friday, the highest number since late March and up from 5,274 on Thursday, government data showed.
The number of deaths remained relatively low, however, with 11 more reported, reflecting the impact of a vaccination campaign that has delivered one dose to three-quarters of adults and two to just over half.
Last month, Public Health England said two shots of COVID-19 vaccine were almost as effective against the Delta variant as they were against the UK variant.
The Delta variant, which is now dominant in Britain according to Public Health England, is thought to spread more rapidly than the previously dominant UK variant, although experts say that vaccines still offer protection against severe disease.
(Reporting by Alistair Smout and Sarah Young; Additional reporting by Andy Bruce and Paul Sandle; Editing by Kate Holton and Grant McCool)