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 to derail that 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. Statistics published on Friday, however, looked to be moving in the wrong direction.

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 remained at over 1 for a second week and the epidemic could be growing by as much as 3% each day.

The estimated R number was between 1.0 and 1.2, meaning that on average, every 10 people infected will infect between 10 and 12 other people. Last week, it was estimated at between 1.0 and 1.1.

The ONS estimates - based on samples of the population - also suggested the UK variant of COVID was no longer the dominant strain in England and that the increase in cases was down to the Delta variant.

Public Health England warned on Thursday that the Delta variant is now dominant in Britain and might have an increased risk of hospitalisation.

It is thought to spread more rapidly than the previously dominant UK variant, although experts say that vaccines still offer protection against severe disease.

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.