Tony Blair has admitted he is powerless to increase access to NHS dentistry.
A new dentist contract starts in April
The government is increasing training places by a quarter and introducing a new contract to improve access - only half the population is registered.
But in an interview on BBC Breakfast News, the prime minister said there was no way of forcing dentists to do NHS work instead of private.
More than a third of English dentists' work is carried out privately - up from about a tenth 15 years ago.
Mr Blair blamed the rise on a new contract introduced in 1990 which effectively increased workload without a rise in pay.
The rise in private work has meant that many people have struggled to find NHS dentists with queues being seen in some parts of the country when a new practice opens.
However, the large number of unregistered people will also include those who have decided not to register themselves.
Mr Blair said dentistry was the "most difficult aspect of the NHS".
"We can't turn the clock back, because I can't force dentists to come back into the NHS."
The admission comes six years after the prime minister pledged to give everyone access to NHS dentistry within two years.
The government is attempting to remedy the situation by introducing a new contract in April which allows dentists to spend more time on preventative work in a bid to get away from the "drill and fill" culture.
The number of training places have also expanded by 25% this year, although it will be five years before those dentists graduate.
Nigel Cater, of the charity British Dental Health Foundation, said he could not see the situation improving in the short-term.
"I don't think you will see a rush of dentists coming back to the NHS, although the new contract may stop them leaving, and the new training places are going to take a few years to make a difference."
And Lester Ellman, of the British Dental Association, said the new contract may "lead to more dentists going private".
My wife is Estonian, so I visit that country at least twice a year. We both use her Estonian dentist and save a fortune. We are happy with the treatment, are always seen on time and it costs less than 1/3 of what we used to pay for private dentistry in this country. It's no good for emergencies, but if I needed major work such as a crown I could pay the air fare and dental costs and still save on going private here.
Richard Simpson, Farnborough, UK
I work for a PCT looking at dental contracts and even I can't get a dentist to look at me!
J. Richards, Birmingham
I am a dentist. I care about the standard of care I can provide for my patients. The NHS does not provide my premises, they do not pay my staff, they do not buy my equipment or maintain it, they do not pay for any of my computers or staff training. I have financed all this myself in order that I can provide the best for my patients. When the NHS pays hardly enough to cover the cost of all the above we are left with only one choice- to leave and go private.
Michael Edgecombe, Norfolk, UK
My local practice set up about 10 years ago and now that it has built up its practice it is abandoning those NHS patients that have loyally supported them over the years. And get this, they are now demanding you pay £30 a year even to register with them as private patient! Pure greed!
I can see the argument that there would be more NHS dentists if the government paid them more, but can I ask you - have you ever seen a poor dentist?
I've been going to my dentist ever since I had teeth and they have now introduced their own private scheme. I never need anything done other than a check up and spend literally 2 minutes in the chair twice a year. Their new service wants to charge me a £66 registration fee and £33 per check up for this. I have had to leave as I refuse to pay this for 2 minutes work. Luckily I have managed to find one NHS dentist nearby but all others weren't taking on new patients.
Sarah Harris, Warwick, Warwickshire
People are saying that private dentists are just greedy, but it takes a lot of time and money to run a good practice. Staff are hard to come by, yet they need to be retained, and the government's made it so that workload has increased and pay has been capped for NHS only dentists.
Nick, South Yorks
My sister left for Australia in the 90s as she couldn't make a living here doing NHS work and didn't have £100,000 or more to buy a share in a practice. If they train more dentists, more will leave the UK for better jobs abroad. It's unfortunate that no-one in New Labour is fit to run a business, otherwise they would understand this.
I have an excellent NHS dentist who has helped me through a lot of treatment despite my extreme dental anxiety. I think it's a shame not everyone has access to this type of service, especially people who are anxious or phobic, as we can find it hard enough to find dentists willing to take us on, without dealing with these issues of shortages.
Cait Hughes, Birmingham, UK
Our NHS dentist whom we have been going to for over 30 years sent us a letter on Monday that from 5pm that evening they no longer would care for NHS patients. We now have to register as private clients in their practice or leave.
Alun Davies, Betws-y-Coed
If the dentists are being taught at taxpayers' expense over 5 or so years, I believe they should be forced to work as an NHS dentist for a fixed period of years. If they decide to go private only, they should be forced to repay the cost of their training. This is only fair to the taxpayer.
R. Brown, London, UK
Why should the government and the NHS pay for dental care? People are prepared to spend thousands of pounds on maintenance of their car for example, but they don't see why they should spend a penny on their own dental maintenance. Our health is our own responsibility, and I do not see why the taxpayer should subsidise those who are not prepared to look after their own bodies. Preventative healthcare is the key, and that is everyone's responsibility.
John Buckley, Reading, UK
Our local dentist has recently retired and being unable to find a dentist to take on his practice, we have all had to try to register with other dentists in the area. We are a town with a population of approx 9000 with one dentist who will not take any more NHS patients and do not have a waiting list for places. The dentist 5 miles away has shut their satellite surgery and moved their dentist back to the next big city which is 17 miles away. We have a right to NHS care but are unable to receive it which I feel is ridiculous and unacceptable in this day and age!
Pamela, Blairgowrie, Scotland
If you look at other EU countries, such as France, they offer free university education to students studying essential subjects, such as engineering, while those studying other non-essential subjects do not receive grants and pay their own fees and costs. We have started doing this with teaching and NHS disciplines, maybe we need to extend this to other essential skills such as dentistry.
Sian van Es, Hereford
Our dentist suddenly no longer takes NHS patients and expects us to join a private insurance scheme at huge expense. What happens to the NHS contribution which presently goes towards our costs? There is no NI reduction.
William Douthwaite, Milnthorpe, Cumbria
If the British population took it upon themselves to boycott private dentistry, dentists would have no choice but to return to the NHS. As long as people are willing to pay ridiculous amounts of money to private dentists they will continue to desert the NHS. Blair is absolutely right, no government can legislate against greed.
I have been unable to find an NHS dentist since moving to Preston one and a half years ago. There simply aren't any within 50 miles. I've asked NHS Direct for help, but they have merely ignored emails I have sent.
Andrew Hindle, Preston, Lancashire
I went to see a private dentist in Leeds who after an x-ray stated I needed 3 fillings for £300, in and out in five minutes tops. I then (after a year searching) got a second opinion from an NHS dentist who gave me a clean and said I didn't need any treatment for about £25! The private dentists are ripping us off!
The government took away my dentist and now I have no teeth, I have to drink everything through a straw and it's DISGUSTING.
Anthony Bentley, Norwich
In all fairness they have encouraged new trainee dentists but to claim to be powerless is nonsense. Basic market forces apply - if you need dentists to work for the NHS... Make it worth their while and pay them on a level that at least "starts" to compete with what they can earn privately.
R. Williams, London, UK