Dentists are facing a crackdown in the way they charge patients for private work amid growing concern over the erosion of National Health Service treatment from dental surgeries.

Mounting evidence that a substantial number of patients are being forced to pay for fillings and dental work when they had assumed it would be done on the NHS has been gathered by the Consumers' Association, which is pressing for a far more open and transparent system of charging.

The Office of Fair Trading has been investigating the dental market and is shortly to produce a report which will highlight the confusion over pricing. It is expected to recommend a more transparent, better regulated pricing system, and also that an independent investigator be appointed to look at patients' complaints.

For the first time this year, dentists' private earnings topped their NHS wages, with 51 per cent of their overall income coming from charges, according to analysts Laing and Buisson. The average dentist earns around £60,000 a year, but there is enormous disparity with some NHS-only dentists taking home around £30,000 while others, particularly in the South East, are earning six-figure salaries.

The enormous confusion over charging methods is revealed in the case of Will Conroy, a 33-year-old freelance journalist who believed he was an NHS patient when he was actually being seen privately.

Conroy had gone to dentist Dr Ian Hancock in Manchester last November, attracted to his Oxford Road practice by the large sign outside: 'All NHS Treatments Offered'.

'There was no mention before I got into the chair that some of his procedures were being carried out privately - I assumed it would be done on the NHS,' said Conroy.' He drilled deep into one of my molars. Then he sat back, and told me I would need a root canal filling.

'He told me: "This is a big job. Normally a private dentist would charge you £600 but I'll do it for you for between £200 and £250." I couldn't believe that I was in the middle of treatment, in some pain, having to think about money. When I said I thought it wasn't being done privately, he said: "There's no way I could do this on the NHS, we're not paid enough."

'I agreed to come back to have subsequent work done and left after paying a deposit of around £20, but in fact I contacted another dentist who did the work on the NHS. It still cost me £130, though.'

But Hancock, whose income mainly comes from NHS patients, robustly defended his decision. 'I sympathise with Mr Conroy and I'm sorry that he assumed that the treatment was on the NHS, but we can't offer that for everyone. I cannot take everyone who comes off the street as an NHS patient.'

He said that the low payments offered for NHS were such that had had to do some private treatment. 'Many of my patients are on income support, but if there are patients who can afford it, they may have to pay. In fact, I charge much less than many of my colleagues.'

Back in 1999, Tony Blair promised to restore NHS dentistry, saying everyone would be able to have access to this by October 2001. When it became obvious that this was unachievable, the pledge was then rewritten, by setting a target that everyone should be able to have NHS dental treatment 'in an emergency'. Despite the setting up of dental access clinics, however, not even that target has been achieved.

Cases like this worry the Consumers' Association, which says that the current complaints procedure makes it extremely unlikely that a patient could ever argue that they had been the victim of coercion.

Last year, it put in a 'super-complaint' to the OFT asking them to investigate a number of areas. The main issues it raised were the lack of price transparency in dental treatment, the complaints handling system and the way dentists were flouting their own profession's guidelines when it came to setting out their charges and writing treatment plans.

Frances Blunden, the association's senior policy adviser, says the present situation cannot be allowed to continue. 'There is a lot of unhappiness over the fact that more and more patients are having to go privately. The situation appears to be getting worse, as people are reporting that they go back to the dentist they have seen for years, only to find that they now only do private work.'

Blunden said that much of the Government's money for dentistry had gone into NHS dental access centres, which provide care on an emergency basis for patients. 'The difficulty is that you can't register with these centres on a long-term basis, which means that they won't pick up problems at an early stage.'

There are around 31,000 dentists in Britain working in hospitals, clinics and practices. Though there are no firm figures, an increasing number appear to be doing less NHS work.

Dentists are allowed to choose how much private work they do. The British Dental Association argues the growth in private care is driven by patients, as the public demands different treatment options.