The rise in infections that will follow the government’s plans to reopen on 19 July will put intense pressure on the NHS and hit the service’s attempts to tackle its huge backlog of care, hospital bosses have said.

They voiced deep unease that the lifting of almost all restrictions, and especially mandatory face masks, would put already busy hospitals under extra strain.

Dr Layla McCay, director of policy at the NHS Confederation, said many GP services and hospitals were already under greater pressure than is usual for the time of year and providing the amount of care they would usually offer in winter. “Already for some of our members, this is feeling unsustainable,” she said.

“The government itself is already warning of 100,000 Covid-19 cases a day in the coming weeks, and new numbers from Sage predict that we could soon see 1,000 hospitalisations a day, which will mean once again that the NHS will come under intense pressure and see the health service struggling to make inroads into the huge backlog of non-Covid treatment as quickly as it would like.

“Messages to go slowly, cautiously and steadily are right, but they are also open to wide interpretation, which is concerning when that interpretation may be reflected in the pressure our health system will experience over the coming weeks.”

Pat Cullen, acting general secretary of the Royal College of Nursing, said: “Public mask-wearing is straightforward and well established – the government will rue the day it sent the wrong signal for political expediency.”

Doctors have voiced their alarm at the potential impact of unlocking on the NHS in recent days. Dr Richard Wenstone, an intensive care consultant in Liverpool, said last Saturday that the city’s hospitals had experienced a big increase in Covid admissions, that it was in its “fourth wave” of cases, and that he and his colleagues were worried about the next few weeks.

“I think myself and a lot of my colleagues are fearful about what might happen,” Wenstone told the Liverpool Echo. “On the basis of what I’m seeing, I would say it’s the wrong thing to do.”

One of his colleagues, Dr Peter Hampshire, tweeted last Friday that isolation by personnel at their NHS trust, combined with colleagues taking summer holiday leave, “is causing carnage with staffing”. That was affecting operating theatres, where surgical teams are trying to tackle the NHS’s huge backlog of care, he added.

He bemoaned the lack of a plan to give the NHS more staff and more money to deal with Covid, and called the 19 July plan “bewildering”.

On the same day Sarah-Jane Marsh, the chief executive of Birmingham Women and Children’s NHS foundation trust, tweeted that the city was once again resurrecting its plans to deal with Covid. “No one has the physical or mental resilience for this wave, but it’s here and so we must find some inner strength to start again,” she said.

Chris Hopson, chief executive of hospitals group NHS Providers, told ministers that unlocking would inevitably limit their drive to cut waiting lists. The total number of people waiting for hospital treatment in England stands at 5.3 million, but the health secretary, Sajid Javid, said on Sunday that it could soon hit 13 million.

Hopson said: “The NHS will need to treat increasing numbers of Covid-19 patients in hospital at a time when the service is going full pelt to recover backlogs, is seeing record emergency care demand for this time of year, is losing significant numbers of staff to self-isolation, and has much reduced capacity due to infection control.

“This will inevitably mean the NHS will be unable to recover care backlogs as fast as trusts and patients would like, and ministers should be clear about this trade-off. In this context, predictions of at least 1,000 Covid-19 admissions a day from the Scientific Advisory Group for Emergencies are concerning.”

Rachel Power, chief executive of the Patients Association, said more hospitals may have to cancel surgery, as at least one main acute trust, in Leeds, did last week.

She said: “The NHS should be given the breathing space to work through the backlog of treatment, which it won’t be able to do if cases escalate because all restrictions on our behaviours are lifted. If the health service cannot make a dent in waiting lists any time soon, that will mean more misery for patients.”