Boris Johnson has warned the lifting of lockdown restrictions “may have to wait” on June 21 amid a concerning rise in cases of the Indian variant of Covid-19.

Cases of the variant have doubled in the last week sparking fresh concerns over stage four of Boris Johnson’s roadmap out of lockdown.

Three-quarters of new cases are now caused by the Indian mutation of the virus.

Ministers are remaining cautious on the prospect of all measures being scrapped in England on June 21, as set out in the Prime Minister’s road map, although hospital admissions remain flat.

Officials are examining the data after confirmed cases of the Indian variant of Covid-19 reached almost 7,000. It is now the dominant strain in the UK, one expert said.

Epidemiologist Professor Neil Ferguson said the planned unlocking next month now “hangs in the balance” due to the growth of the variant of concern.

The Prime Minister told reporters on Thursday he “didn’t see anything currently in the data” to divert from next month’s target, adding: “But we may need to wait.”

Health Secretary urged the public to remain vigilant in Thursday’s Downing Street press conference.

Mr Hancock said the increase in cases of the Indian variant remained focused in “hotspots” where surge testing and vaccinations were taking place.

He added that of the 49 people in hospital with coronavirus in Bolton, only five have had both doses of vaccine.

“So when you get the call, get the jab, and make sure you come forward for your second doses so you can get the maximum possible protection,” he said.

“The vaccine is severing the link between cases and hospitalisations and deaths from coronavirus.”

Epidemiologist and Government adviser Professor John Edmunds also warned on ITV’s Peston this week that it looked “a little bit risky” to be relaxing all restrictions in just over three weeks time.

Dr Jenny Harries, chief executive of the UK Health Security Agency, said she agreed with Prof Ferguson’s reading of the situation and admitted the numbers had become “quite worrying”.

“If you just look at the pure data which is out today it looks quite worrying,” she told a Downing Street press conference.

“We had 3,535 cases of the 617.2 last week, and we have just about double that, 6,959, now.”

She said it was “on the cusp at the moment” over whether rising cases reflected the variant taking off or whether there was a rise because more cases are being hunted for and detected, with more socialising also now permitted.

“The good news, of course, is we are not seeing that generally translate into increased cases of hospitalisation and definitely not into deaths,” Dr Harries added.

“So the key message there is … if we can hold it while the vaccination programme gets rolled out, we stand a much better chance of getting through this session.”

Public Health England (PHE) put the hospital admission rate for Covid-19 at 0.79 per 100,000 people in the week to May 23, compared to 0.75 per 100,000 in the previous week.

In England, 6,180 cases of the Indian variant have been confirmed, along with 702 in Scotland, 58 in Wales and 19 in Northern Ireland.