NHS bosses are planning a massive expansion of the controversial rationing that forces smokers and obese patients to wait months in pain before they can have surgery, a leaked letter reveals.

The move will see local NHS bodies across England implement restrictions on access to treatment that will hit what doctors’ leaders believe will be tens of thousands of patients.

The plan is disclosed in a letter sent on 15 March by Dr David Black, NHS England’s medical director for Yorkshire and the Humber, to Rotherham Clinical Commissioning Group (CCG). In it, he praises the GP-led group that controls the NHS budget for treating the town’s residents for introducing what critics call “lifestyle rationing”, which compels dangerously overweight patients or those who smoke to wait for hip or knee surgery. “We are very supportive of your work to best manage resources for the benefit of all patients and understand that this may mean that difficult decisions need to be made,” Black wrote.

And he added: “We expect that many CCGs will be in the process of developing similar schemes and initiatives to deliver plans for 2017-19. This is something we would encourage, where plans are well developed and clinically validated.” He then told the CCG to give him four weeks’ warning of any further plans “to change access thresholds” so that NHS England can prepare for the fallout.

The move has triggered a storm of protest, with ex-health minister Norman Lamb warning that condemning patients to long, painful delays for care is destroying the NHS’s fundamental principles and obliging those affected to pay for private treatment.

“This is yet more evidence of the creeping advance of rationing. Guidance based purely on medical judgment on weight loss is fine, but what is happening around the country goes well beyond that in practice. It will inevitably result in those people with money paying for speedy treatment, while everyone else is left waiting,” said Lamb, who is the Liberal Democrats’ health spokesman.

“We are seeing, bit by bit, the destruction of the solidarity that this country has been so proud of with the NHS – the idea that whatever your income or wealth, you get access to the care you need, in your hour of need,” he added.

Labour claimed that underfunding of the NHS lay behind the extension of rationing. “This secret memo from NHS chiefs reveals the truth of what’s happening to the NHS under the Tories – more and more rationing of treatments. People will be waiting longer and longer in pain and discomfort for surgery such as hip and knee replacements,” said Jonathan Ashworth, the shadow health secretary. “There is now a very clear choice in this election. Cuts, longer waiting times and restrictions on treatment under the Tories, or Labour, who will return our NHS to its founding principle of universal provision, free at the point of need, with best quality of care for all,” he added.

As many as one in five patients awaiting a hip or knee operation ends up being denied surgery for months after such schemes are introduced, it emerged last week. Seventy-nine out of 408 patients needing a knee replacement in Scarborough and Ryedale, and also in Yorkshire, have not been referred to hospital for the procedure since the CCG brought in that policy last November. The 79 have been told to lose weight or stop smoking during the delay – which the CCG calls “a six-month period of health optimisation” – before they have their surgery.

The figures, uncovered by the Yorkshire Post, led the Royal College of Surgeons to claim that “it is disgraceful that one in five patients is having … surgery delayed for at least six months based on what are, frankly, arbitrary criteria.”

Ian Eardley, the college’s vice-president, told the Observer: “It is deeply concerning that one in five patients are now being denied immediate surgery because they smoke or are deemed obese. There is no medical evidence to justify restricting referrals for entire groups of patients; it is effectively a form of discrimination. These figures are just the tip of the iceberg as we know that over a third of commissioning bodies are pursuing similar policies, meaning tens of thousands of patients across the country are facing restrictions on their surgery.”

Patients denied hip or knee surgery can end up in severe pain or have trouble walking, “making it near impossible for them to lose the required weight to allow them to access surgery in the future. Patients need all of the support they can get to lose weight or to stop smoking, but removing their rights to timely NHS surgical care will obviously not achieve this,” he added. Eardley called on all political parties to make it clear during the election campaign that they do not think the NHS should impose such restrictions on those who are obese or who smoke.