Private practice directly affects the quality of care that NHS patients receive and doctors should not be allowed to work "on both sides of the divide", writes a senior doctor in The BMJ this week.
John Dean, a consultant cardiologist at Royal Devon and Exeter NHS Foundation Trust Hospital, describes how he quit private practice after realising "it has direct adverse effects on the NHS."
To begin with, he says he felt that he needed the money "to renovate the house, educate the children, and so on." And he was sure that he could keep the private work separate from the NHS work. But, he says, "it became increasingly difficult to keep the lid on the private jar as the contents expanded, and spillage was inevitable."
The fact is that the business of medicine and the practice of medicine are at odds, he argues. Private medicine encourages doctors to make decisions based on profit rather than on need.
"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 for which the conduct of some involved was so venal it bordered on the criminal - the greedy preying on the needy," he says.
He believes that private work has direct adverse effects on the NHS. A consultant cannot be in two places at once, he writes, and time spent in the private sector deprives the NHS of this valuable resource.
And he points out that, although patients think they are paying for higher quality medicine, the main advantage is simply to jump the NHS queue. "Private hospitals are five star hotels but for the most part no place to be if you are really sick."
But the most pernicious aspect of private medical work, he says, 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."
Private practice also creates a perverse incentive to increase your NHS waiting times, he adds.
The inescapable fact is that money is at the root of it all, he says, which is why he left private practice and why he believes the rulers of healthcare "should draw an uncrossable line between private and public medicine and tell doctors to choose: you cannot work on both sides of the divide."