Private healthcare is a "con" and NHS doctors who boost their incomes by carrying it out are depriving the public sector of a valuable resource, a cardiologist has said.
Writing in the British Medical Journal, John Dean suggested doctors be told to choose between the two rather than working on "both sides of the divide".
He said he became "increasingly uncomfortable" with working in private practice and decided it would be against his conscience to continue.
"No matter how high I set my own moral and ethical standards I could not escape the fact that I was involved in a business where the conduct of some was so venal it bordered on criminal - the greedy preying on the needy," he wrote.
'Direct adverse effects'
Dr Dean, a consultant cardiologist at Royal Devon and Exeter NHS Foundation Trust Hospital, said working in private practice "has direct adverse effects on the NHS" by taking up doctors' time.
"The business of medicine and the practice of medicine are at odds," he went on.
"Private medicine encourages doctors to make decisions on the basis of profit rather than need. When confronted with a choice between two treatment pathways in equipoise - one that earns the doctor no money and the other with a fat fee attached - that conflict is stark.
"I cannot say, with hand on heart, that I have never chosen the second option."
Likening private healthcare to being swayed into buying expensive shampoo, he said the "promise is far greater than the reality".
"Private hospitals are like five star hotels, but for the most part they are no place to be if you are really sick."
He said the most pernicious aspect of private medical work is the indirect effect it has on a consultant's NHS practice.
"It is difficult to justify subjecting private patients to unnecessary tests and treatments if you avoid doing them to NHS patients. So you have to operate the same system in both wings of your practice to ease the stress of this cognitive dissonance."
There should be no conflict
Dr Dean said private practice creates a perverse incentive to increase your NHS waiting times - "after all, the longer they are, the more private practice will accrue".
The British Medical Association (BMA) said it was difficult to provide a clear figure for how many NHS doctors are also involved in private work as it is not officially recorded.
A spokesman said: "There should be no conflict of interest between NHS and private work, and this principle is contained in consultants' employment contracts.
"Consultants who want to do private work must first offer to do extra work for the NHS, ensuring NHS work is the priority."