Pressure is intensifying on Boris Johnson to delay England’s planned release from lockdown restrictions on 21 June, after new figures showed rising infections and hospitalisations and scientists warned premature relaxation could lead to a dangerous spike in Covid-19.

Concerns were fuelled by the announcement of 4,182 lab-confirmed Covid-19 cases in the UK — the highest daily number since the end or March — as well as new estimates that the all-important R rate of reproduction of the disease has now risen back above the crucial level of 1, meaning the virus is spreading exponentially.

Downing Street insists that it is too early to say whether the prime minister will be able to give the go-ahead on 14 June for the removal of most remaining curbs a week later, after he warned on Thursday that the country may have to wait longer.

But a string of experts raised the alarm over the timetable set out in the prime minister’s roadmap, with one saying that the 21 June date is now “certainly in doubt” and another warning that things could “get bad very, very quickly” if Mr Johnson lifts curbs too early.

Scottish first minister Nicola Sturgeon delayed a decision on releasing Glasgow from the highest level of restrictions until at least the end of next week, warning that case numbers in the city are “uncomfortably high”.

Professor Andrew Hayward of University College London, a member of the government’s New and Emerging Respiratory Virus Threats Advisory Group (Nervtag), said cases of the Indian variant were doubling every week and “it only takes five or six doublings for that to get up to a quarter of a million cases.”

He said if further easing was brought in “instead of doubling every week it’s likely to double more frequently than that of course, so I think there is a good argument for caution until such time as we’ve got a much higher proportion of the population double-vaccinated”.

Office for National Statistics data showed an estimated one in 1,120 people in private households in England had Covid-19 in the week to May 22 – broadly unchanged from one in 1,110 in the previous week. The estimate for Scotland was around one in 630, up from one in 1,960, putting Scotland back to where it was around a month ago.

The R value for England rose from between 0.9 and 1.1 last week to between 1 and 1.1 now. Ms Sturgeon said the level in Scotland could be as high as 1.3.

The scientist credited with warning the government to abandon its “herd immunity” strategy in the spring of 2020, Cambridge professor Sir Tim Gowers warned that “things will get bad very, very quickly” if ministers misjudge the timing of the last step out of lockdown. “The downside of being a bit more cautious is quite a lot smaller than the downside of getting it wrong,” he said.

And UCL professor Christina Pagel suggested a delay until the end of July before the roadmap’s step 4, which will usher in the removal of legal limits on social contacts, reopening of nightclubs and lifting of restrictions on attendance at events from weddings to sports fixtures and rock festivals.

“If we can just delay international travel, delay stage 4 of the roadmap until we have a much higher proportion of people vaccinated with two doses, we’re in a much, much better position,” she said. “We’re only two months away from that, it’s not long to wait. What I don’t want is for us to have new restrictions.”

University of East Anglia professor of medicine Paul Hunter said that even if the planned relaxation goes ahead on time in three and a half weeks, further restrictions could be reintroduced later at a local, regional or national level.   

And he warned that another flare-up of the virus in the autumn could require “the continuation of some types of social distancing perhaps until the next new year”.

“The next easing of lockdown in June is certainly in doubt,” said Prof Hunter. “That doesn’t mean to say it won’t happen, but there’s a lot of uncertainty around that.”

Industry figures warned that any further extension of lockdown restrictions could be crippling for business.

A survey for the Night Time Industry Association found that nine in 10 nightlife operators fear any delay to the full re-opening of hospitality could threaten the survival of their businesses.

NTIA chief executive Michael Kill said that any slippage in the timetable would “wreak havoc” on the sector, explaining: “Businesses have already had to engage staff, order stock, book entertainment and sell tickets, all at a considerable cost. Evidently, the government has a misguided understanding of the timelines, preparation and operational requirements of our sector, given the ridiculous seven-day notice period they have said they will give ahead of reopening.” 

Although figures showed a 25 per cent increase in hospitalisations to 860 over the past week, experts said that the key to determining whether the 21 June relaxation can go ahead will be whether vaccines continue to prevent mild cases developing into serious illness and death.

Prof James Naismith, director of the Rosalind Franklin Institute, said that the variant B1.617.2, first identified in India, had now spread from initial hotspots in Bolton and Blackburn and “reached pretty much everywhere”.

“It seems almost certain that we will face a third episode of rising Covid-19 infections,” he said. “The questions are how large will the peak number of infections will be and how much harm will this third episode do.”

And University of Oxford professor Andrew Pollard, the chair of the Joint Committee on Vaccination and Immunisation, said more time was needed to see how effective the jabs were in protecting the most vulnerable from the new variant and confining any infections to mild symptoms.

“We are still waiting to have the further understanding of the degree to which, with this variant, the vaccines are able to unlink for the vast majority of people the risk of severe disease,” he said.

“Infection itself is something which we can tolerate. What we can’t tolerate are large numbers of people going into hospital.”

Dr Helen Wall, senior responsible officer for the Covid vaccine programme in Bolton, said there were signs that people coming into hospital now are not as badly affected as those who fell ill in earlier waves of the disease.

“In terms of how ill they’re getting, I think the vaccine definitely seems to be working,” she said. “We are not seeing, certainly not many people as sick as we would have done pre-vaccine.”

Business secretary Kwasi Kwarteng said there was currently “nothing in the data” to suggest that the earliest date for removal of restrictions should be moved from 21 June.

But he added: “The caveat obviously is the data can change. So if scientific evidence data points to an increased hospitalisation rate, increased degree of risk, then we have flexibility to move that date.”