By Nick Triggle
BBC News, health reporter
NHS dentists should face quotas forcing them to do more NHS work rather than private treatments, campaigners say.
Some 2m people cannot get access to NHS dentistry
Patient Concern said quotas should be imposed as figures show that for the first time under 50% of NHS dentists' income is now from NHS work.
Labour MP Kevin Barron, of the Commons health committee, said dentists had a "moral obligation" to give the NHS more as it costs £175,000 to train them.
But dentists said quotas would be "unfair and unworkable".
According to latest figures from the NHS Information Centre, NHS dentists on average now earn about 52% of their income from private patients - the first time private work has accounted for a greater share.
That compares with 42% in 1999-2000 and the rise has helped push average earnings up to £86,000 - a rise of 11% in a year.
An analysis of figures from the General Dental Council register also suggest the number of dentists treating solely private patients has soared by over a third in the last two years.
There is no exact figure for how many are doing this, but it is estimated to be between 2,500 and 3,500.
This compares with the 21,111 dentists with NHS contracts - a slight increase on previous years - but a figure which still leaves 2m people without access to NHS care.
Mr Barron, who chairs the Commons health committee, said: "I think [dentists] are letting the country and taxpayer down.
"The profession should take a good look at itself. [Dentists] just do not have the same public sector ethos that GPs do for example."
Joyce Robins, co-director of Patient Concern, said: "Access to NHS dentistry has been a long-running problem, I don't see why we can't impose quotas on dentists who work in the health service to ensure they do more NHS work."
She said this quota should be imposed on new dentists for several years before they could opt out, although dentists should still be allowed to do some private work.
Professor Jimmy Steele, who teaches dental students at the University of Newcastle, said a quota on everyone would not be fair on dentists who had perhaps spent a lot of their career in the NHS.
But he agreed new dentists could be tied in to a certain amount of NHS work for a period after qualifying, suggesting five or 10 years.
"They [dental students] are heavily supported by the taxpayer through long and expensive training so do have a duty to the NHS, but when I put that to my students they are sometimes surprised by the notion."
But Lester Ellman, of the British Dental Association, said quotas would be "unfair and unworkable".
"The reason dentists are turning towards private work is that they have been unhappy with the way the NHS [system] has worked for years.
"They do not get time to treat the patients properly."
A spokeswoman for the Department of Health said: "It would not be reasonable to prevent dentists carrying out private work if they are meeting their NHS commitments."
She added a new system introduced last year was designed to give the NHS more funds if dentists left the NHS.