NHS bosses have urged hospitals to send patients to be treated by private healthcare firms in a bid to reduce the increasing number of patients waiting for planned operations.

The move has sparked claims that it will waste scarce NHS funding and that profit-driven operators will use the service’s desperation to cut waiting lists to charge higher prices.

The total number of patients in England on the waiting list – for procedures such as hip or knee replacements or cataract removals – rose to 4.3 million in June, the highest figure for a decade.

Q&A What are the financial pressures on the NHS that have built up over the last decade? Show Between 2010-11 and 2016-17, health spending increased by an average of 1.2% above inflation and increases are due to continue in real terms at a similar rate until the end of this parliament. This is far below the annual inflation-proof growth rate that the NHS enjoyed before 2010 of almost 4% stretching back to the 1950s. As budgets tighten, NHS organisations have been struggling to live within their means. In the financial year 2015-16, acute trusts recorded a deficit of £2.6bn. This was reduced to £800m last year, though only after a £1.8bn bung from the Department of Health, which shows the deficit remained the same year on year. Read a full Q&A on the NHS winter crisis

Many hospitals are finding it impossible to treat 92% of patients within the supposed 18-week maximum waiting time, which is one of the NHS’s key performance targets.

NHS England and NHS Improvement have written to hospitals and clinical commissioning groups (CCGs) telling them to produce plans urgently to tackle the growing backlog. The letters suggest that that may involve private firms being paid to operate on patients whom NHS hospitals are too busy to treat.

In his letter Matthew Swindells, NHS England’s national director, told the 195 CCGs, the local NHS bodies that pay for care, that waiting lists are now so huge that patients are having to wait for “inexcusable” lengths of time to receive the care they need.

As a result, he told them: “Where trusts determine that they will no longer be able to meet their activity and performance commitments in their board approved plan you should work with your trusts to determine how these gaps will be closed through use of capacity in other trusts and/or the independent sector.

“Any contingency plan for work carried out by other trusts or the independent sector should be available to mobilise by mid-September.”

The edicts are likely to lead to an increase in the number of NHS patients having surgery provided by firms such as Care UK, Spire Healthcare and Virgin Care, but paid for by the NHS.

The move has been prompted in part by the NHS’s unprecedented cancellation last winter of non-urgent operations in order to ensure hospitals had enough beds to cope with illness linked to the cold spell, which in turn led to the waiting list ballooning.

The Royal College of Surgeons said bringing in the private sector was “necessary in the short-term to benefit patients”, given the number of people waiting. Over 500,000 of the 4.3 million patients have already had to wait more than 18 weeks.

But an RCSE spokesperson added: “Unfortunately, private hospitals are likely to charge a premium at such notice and they are unlikely to make a significant dent in the waiting list given their own capacity is constrained by a rise in people self-paying for surgery. So this is little more than a sticking-plaster solution.”

NHS Providers, which represents trusts, voiced unease about the plan, which it warned could undermine hospitals’ already fragile finances.

“We would caution against sending a large amount of work to the private sector. All this will mean is taxpayers paying a premium rate for an NHS surgeon to do a procedure that most could be doing at an NHS trust. As well as wasting taxpayers’ money it could also further de-stablise the provider sector’s already challenged finances”, said Chris Hopson, its chief executive.

Jonathan Ashworth, Labour’s shadow health secretary, criticised what he said would be a “bonanza for the private sector”.

“It’s a total indictment of this government and their funding settlement that NHS bosses now feel they have no option but to pay for operations in the private sector. But patients and taxpayers deserve quality, good value, care not the premium prices private hospitals will charge our cash-strapped hospitals.

Kevin Brandstatter, GMB National Officer, said the service had been forced to turn to private providers because it had been denied proper funding.

“This is not a solution to sorting out the ever growing waiting lists. Rather than divert public finance to private companies only interested in profit, the NHS should be properly resourced and given the money it needs to provide proper patient care,” he said.

NHS Partners Network responded to say that the cost will be the same to the taxpayer, regardless of who carries out the procedure. David Hare, the network’s chief executive, said “For the last two decades, the NHS has used private sector capacity to cut elective waiting times – delivered at NHS tariff prices, to NHS standards and free at the point-of-use for patients.”

• This article was amended on 3 September 2018 to include a quote from NHS Partners Network, and to correct Kevin Brandstatter’s job title.