More than 21,000 patients have been waiting longer than government target time for non-urgent treatment at Milton Keynes hospital

And the Royal College of Surgeons of England warned there could be a backlog "double whammy" following the crisis, when new patients who come forward as the pandemic eases are combined with those already waiting for care.

According to NHS rules, patients referred for non-urgent consultant-led elective care should start treatment within 18 weeks.

But NHS data shows 100 per cent of patients on the waiting list for elective operations or other treatment at Milton Keynes University Hospital NHS Foundation Trust at the end of April had been waiting for more than 18 weeks – up from nine per cent in the same month last year.

It means 21,807 patients had been waiting longer than the target time.

NHS trusts are normally expected to make sure no more than 8% of patients are left waiting beyond the 18-week maximum target.

But non-urgent elective operations – such as hip and knee replacements – were suspended from mid-April to free up beds for coronavirus patients, leading to delayed care for many patients across England.

Nationally, 29 per cent of people were still waiting for treatment after 18 weeks in April – the highest proportion for a single month since April 2008.

That is despite the number on the waiting list falling to 4 million, down from 4.3 million in the same month last year.

Professor Derek Alderson, president of the Royal College of Surgeons of England, said the NHS has coped well with the Covid-19 crisis, but has had to "hollow out" its routine care to do so.

“Patients who have been waiting through the pandemic will very often have been in pain, and the longer some of them wait, the worse their conditions become," he said.

“Elective operations cover not only essential orthopaedic work – giving relief to people in need of new hips, knees and other joints – but life-saving treatment for cancer, heart problems, and neurological disorders."

Professor Alderson added that the NHS will face a "herculean task" to get through the waiting list of new and existing patients following the crisis.

The latest waiting time figures come as the effect of the pandemic on a number of other NHS services were also revealed.

A&E attendances at hospitals in England were down 42 per cent in May compared with a year ago, while urgent cancer referrals fell by 60 per cent in April.

Nigel Edwards, chief executive of the Nuffield Trust health think tank, said: "The latest figures dramatically illustrate the lasting impact that the nationwide lockdown and the extraordinary measure taken by the health service to treat people with Covid-19 will have on patient care."

"Operating alongside Covid-19 measures will mean no return to business as usual. The health service will simply not be able to catch up by working in the way it used to.

"Increased infection control measures, social distancing and practical constraints to protect staff and patients will mean we will see the waiting list grow rapidly over the autumn."