Some dentists are misleading their patients about their right to NHS treatment so they will pay for private treatment instead, according to the Office of Fair Trading (OFT).
It carried out a study of NHS and private dentistry in the UK.
The OFT found that about 500,000 people a year were given the wrong information about their treatment options.
Overall, however, the OFT found that most patients were satisfied with their dentists' services.
In response to the report Health Minister Lord Howe said: "We welcome this study, which has found that the vast majority of patients are happy with their dental treatment - and that the vast majority of dentists behave ethically.
"However, denying patients care on the basis of misinformation is a very serious matter - any dentist that does this risks breaching their contract and we would expect the local NHS to take action."
The OFT discovered several important problems with the availability of dentists' services and the choices for patients.
The regulator concluded that the supply of dentistry in the UK was in need of urgent reform.
"Our study has raised significant concerns about the UK dentistry market which need to be tackled quickly in the interest of patients," said John Fingleton, the OFT's chief executive.
"All too often patients lack access to the information they need, for example when choosing a dentist or when getting dental treatment.
"We also unearthed evidence that some patients may be receiving deliberately inaccurate information about their entitlement to NHS dental treatment, and we expect to see robust action taken against such potential misconduct by dentists," he added.
Clear information 'lacking'
Among other problems, the OFT concluded that:
- Patients should not be pressured by dentists into buying insurance policies to cover the cost of private dental treatment. Twenty per cent of patients who had bought these plans had done so under pressure.
- The restrictions on patients making direct appointments with dental hygienists, therapists and technicians should be removed by the General Dental Council. There were no good reasons for these restrictions, the OFT said, and they limited patients' choice.
- In England, the Department of Health should make it easier for dentists to set up new practices or expand existing ones. Dentists with an NHS practice were effectively protected from competition.
- The complaints system should be made "simpler, easier and less time-consuming for patients and dentists to resolve complaints". The OFT found that many patients did not have their problems sorted out properly if their dental treatment was poor.
The OFT found that nearly £6bn was spent on dental treatment in 2009-10, with 58% being spent on NHS treatment and 42% on private treatment.
However, clear and accurate information for patients was lacking, it found.
It said that 39% of patients at NHS dentists in the past two years had not seen any leaflets or posters telling them what their NHS charges should be.
Fifty-six per cent of dentists that offered private treatment did not show their fees at their reception desks and 82% of patients who had recently paid for treatment had not been given a written treatment plan.
Amanda Hill from Dewsbury told the BBC: "My husband visited our NHS dentist to enquire about getting a bridge on his bottom teeth, he was told that it would have to be done privately."
"I phoned the reception and asked why he couldn't have it done on the NHS, she said he could, but he would have to come back and ask the dentist about it.
"Why was he not given that option in the first place?" she asked.
The OFT singled out the dental authorities for failing to take action against dentists and dental practices that did not follow the rules.
"Effective, timely, sufficient enforcement action against dentists and dental practices is not being prioritised and pursued by NHS commissioning bodies, the Care Quality Commission (CQC) and the General Dental Council (GDC)."
On the half a million people who had been misled into paying for private care when they might have been eligible for free or cheaper NHS treatments, the OFT was even more critical.
"NHS commissioning bodies and the GDC need to be far more proactive in identifying and pursuing formal, robust and timely enforcement action against such instances of misconduct where appropriate."
The Department of Health is already looking at reforming its contracts for dentistry practices.
Health Minister Lord Howe said: "Since May 2010 an extra one million people are now seeing an NHS dentist, this is why we support the study's finding that patients should be able to have direct access to the appropriate dental care professional."
The GDC said it was already planning to introduce new rules from next year requiring dentists in the UK to display their private charges adequately. The BDA said it would publish a code of practice for dentists selling dental insurance policies.