The Royal College of Surgeons (RCS) has condemned NHS trusts in England for changing the criteria for operations, leading to some patients being taken off operation waiting lists.
Some trusts have "re-categorised" patients as no longer needing surgery, a BBC investigation has found.
RCS president Prof Norman Williams says the practice is "outrageous" and "worrying".
Health Secretary Andrew Lansley says treatment should not be rationed.
Under the NHS constitution no patient in England should wait longer than 18 weeks for treatment after referral from their GP, and NHS trusts face heavy fines for having too many people waiting longer than the set target.
However, BBC Radio 4's File on 4 programme has discovered that in some areas of England patients initially assessed as needing operations have subsequently been re-categorised as no longer in need of surgery - meaning they effectively vanish from waiting list statistics.
Among such patients is 20-year-old Kathryn Fallaize, who, suffering from agonising pain with a gallstone, was put on the waiting list for an operation at Maidstone Hospital in Kent in January 2012.
After 18 weeks she received a letter saying she was being taken off the list because her symptoms had been reviewed and she did not meet the new clinical criteria.
Another patient was told she was at the top of the urgent list for treatment, but when her husband called to check why she had not received an appointment he was told new criteria meant she no longer qualified.
This is because Kent and Medway Primary Care Trust introduced new rules for gallstone patients, meaning they must have at least two attacks of acute pain, resulting in a trip to hospital to qualify for an operation.
This change resulted in 250 patients waiting for surgery suddenly being dropped from the waiting list.
In response to the change, Prof Norman Williams of The Royal College of Surgeons told the BBC: "I would doubt whether a patient with gallstones was put on an operating list who didn't deserve to be on that operating list.
"I'd like to know from that particular trust why patients that were put on a waiting list - presumably by a consultant surgeon - had been taken off the list. I find that very, very worrying actually."
He added: "To leave that patient, not to mention in awful pain, but at the risk of developing severe complications because she comes in as an emergency, to me is outrageous."
Kent and Medway PCT has apologised for cancelling the operations but medical director Dr James Thallon insists it was in the patients' best interests.
He says the end result was that the trust saved money - but it was a clinical decision by specialists not to offer unnecessary operations to patients who only have a single attack.
"We can't get away from the fact there is a £20bn productivity problem in the NHS over the course of the next five or 10 years and ultimately we need to meet what is growing demand at a time in which the resources available to the NHS are flat."
Kent and Medway is not the only primary care trust involved in introducing new criteria for specific medical conditions.
Inconsistency across country
Paul McCready saw his GP in March 2010 and was referred to a hospital in Bury in June the same year.
Mr McCready was told he needed a varicose vein operation but when he went back to his GP in late 2011 to ask when the surgery would go ahead, he was told he had been dropped from the waiting list without being informed.
Heywood, Middleton and Rochdale Primary Care Trust, which funds the operations, told the BBC: "As healthcare commissioners we have a duty to ensure that the public's money is spent effectively on patient care.
"Less complex health procedures are provided by other organisations."
Prof Williams believes many PCTs are rationing treatment and cutting their waiting lists by raising the threshold to get onto them:
"The problem is the thresholds vary across the country, so in one place you can have very severe pain before you qualify to have a hip or knee replacement. In another part of the country it's a reduced pain score.
"If patients aren't getting on the waiting list in the first place, then obviously they're not going to show up on waiting lists."
Health Secretary Andrew Lansley says PCTs should not be putting limits on treatment.
"I don't think it's right in any sense that the NHS rations treatment in quite the sense that you describe," he said, speaking to File on 4.
"I have made it clear, my colleagues have made it clear, and the NHS people should be clear, that you cannot have blanket bans on treatment. You have to assess every patient on their merits."
Mr Lansley added: "In the past... there has been what are effectively bureaucratic rules, the purpose of which is to try and limit the number of referrals.
"Primary care trusts have done that in the past, one or two have done it more recently.
"We are very clear that is not the basis on which referral should be undertaken - they should be undertaken on the basis of a judgement of clinical priority."