Boris Johnson faces a difficult decision about whether to end England’s lockdown on June 21 with hospitals already under “worrying” pressure, a health chief has warned.

The continued spread of the Indian coronavirus variant has cast doubt on the ability to scrap restrictions, with ministers considering plans to keep some measures – such as the continued use of face masks and guidance on working from home – in place.

NHS Providers chief executive Chris Hopson said hospital bosses were concerned about the transmissibility of the B1617.2 variant and the large number of people who have still to receive doses of the vaccine.

He warned that although hospitals were not expecting to be overwhelmed by a surge of Covid-19 cases, they were already stretched by going “full pelt” on dealing with the backlog of cases built up during the pandemic and urgent care needs.

Hospitals are operating under reduced capacity due to Covid-19 restrictions and increased numbers of coronavirus patients will add to difficulties by requiring wards to be reconfigured, he added.

But he said it was “very significant and important” that the evidence suggests the success of the vaccination campaign means much lower levels of hospital admissions, serious illness and death than previously experienced.

“Significant numbers of Covid-19 hospital inpatients will adversely impact care backlog recovery,” he said.

“(The) current degree of pressure on hospitals is worrying especially since we saw clear summer demand surges in the two years before Covid-19.

“But if, as evidence above suggests, success of (the) vaccination campaign means much lower levels of hospitalisation, serious illness and mortality, even with (the) new variant, that is very significant and important. This means there is a difficult decision to make for June 21.”

He also suggested the Government needs to consider the increased burden on hospitals in UK holiday hotspots in coming months with people not travelling abroad, saying one trust chief on the south coast had warned they will “struggle” to meet the “significant extra demand”.

Dr Mike Tildesley, a member of the Scientific Pandemic Influenza Group on Modelling (Spi-M) Government advisory panel, said there was still “quite a lot of uncertainty” around the June 21 date.

He told LBC: “We are starting to see signs of course that cases are going up, but at the moment we’re still obviously reporting hospital admissions and deaths at very low levels.”

He highlighted there can be a delay of a week or two between case increases and hospital admissions possibly rising, but said the country was in “a very different place” to where it was in January.

He said scientists had “an awful lot of work to do” to analyse data on the link between cases and hospital admissions, while bearing in mind the situation with the Indian variant, to give evidence to the Government.

And he warned that if a big wave of cases was allowed to build, that could give rise to new, more dangerous, mutations.

“The problem is, if you have huge numbers of cases, then that increases the risk of the virus mutating, and it may be that you might get a variant emerging that all of a sudden evades the vaccines completely.”

Vaccines minister Nadhim Zahawi said the Government would wait for the latest data on June 14 before deciding whether to proceed the following week.

He said the Government aimed to offer two doses of vaccine to all over-50s by June 21 but he told Times Radio “I could do with more supply” because “I will be able to protect more people more rapidly”.

Asked whether the remaining restrictions could be eased if cases were still increasing, he told BBC1’s Andrew Marr Show: “What I’m saying to you is we have to be cautious. We have to look at the data and share it with the country.

“Are we still vaccinating at scale? Big tick. Are the vaccines working? Yes. But are infection rates too high for us to then not be able to proceed because there are too many people getting into hospital? I don’t know the answer to it.

“But we will know it on, hopefully on the 14th, a few more weeks.”

He said that as the virus becomes endemic “we’re going to have to live with a certain amount of Covid being transmitted”.

Pressed on whether there could be a partial lifting of restrictions, with mask-wearing and working from home continuing, Mr Zahawi said: “We need to look at the data.”

We all want to unlock on June 21 but the single biggest threat to that is the Government’s incompetence

Communities Secretary Robert Jenrick was more upbeat about June 21, telling the Sunday Telegraph: “The Prime Minister will ultimately make the decision… but he’s been clear that at the moment, it appears as if the data is moving in the right direction, and we should be able to proceed with the next stage of the road map.”

But Chancellor Rishi Sunak told the Mail on Sunday: “We have to look at the data as it comes in. We will know more as we approach the date.”

Labour leader Sir Keir Starmer suggested the biggest risk to easing the lockdown on June 21 was “incompetence” in the Government.

He said “weak, slow decisions” by the Government on border policy had allowed the Indian variant to spread.

Current data suggests that although hospital admissions are rising in some parts of the country affected by the Indian variant, they are at a very low level compared with the winter peak.

Between May 19 and 25, 870 people went into hospital with coronavirus, an increase of 23.2% compared with the previous seven days.

Meanwhile, the reproduction number – the R value – for England is 1 to 1.1, up from 0.9 and 1.1 the previous week, suggesting the epidemic is growing.