Consumers could be paying NHS dentists more than they need to because practices are not clear enough about their prices and the treatments patients are entitled to, according to a watchdog.

Existing rules state that dentists must have a price list prominently on display in their surgery and set out treatment costs upfront.

But Which? found that 51% of people who visited a dentist in the last six months did not see a price list and one in five (22%) were not clear about charges ahead of their treatment.

In a separate investigation, just half of undercover researchers who visited practices offering both NHS and private treatment saw a price list on display, while the majority did not show any private prices.

Which? said this made it difficult for people to compare costs between practices and between NHS and private treatments.

An expert panel which was asked to assess the quality of the undercover visits found problems with communication of prices and treatment options.

Half of the 25 visits were rated poor or very poor for explaining the difference between NHS and private options, eight were found to be poor or very poor for explaining prices and the same number were poor or very poor for explaining treatment options.

The watchdog's survey also found evidence that some people could be being overcharged, with one in five NHS patients (19%) reporting that they paid more than one charge for one course of treatment over the last two years when they should not have.

The poll found a quarter of people (26%) were unsure about how NHS and private treatments differed and 40% were unaware that all clinically necessary treatment should be provided by the NHS.

Which? said it was therefore important that dentists were clear with their patients about cost and treatment options, noting that 31% of people who paid said costs had put them off having treatment.

Patients who attend a dentist which offers both NHS and private options and need a treatment should not be asked to pay for it privately, although the dentist should explain suitable private options for consideration.

Dentists are not allowed to refuse any treatment available on the NHS but then offer the same treatment privately.

Patients should only be asked to pay one charge for a course of treatment, even if they need to visit the dentist several times.

Which? has launched the Clean Up Dental Costs campaign and is calling on NHS England and regulators to ensure all dentists comply with existing rules and make information on prices clearly available, explain the treatment options properly and make sure patients know whether or not their treatment is available on the NHS.

Which? executive director Richard Lloyd said: "A visit to the dentist is an essential health check for millions of people across the country.

"Most of us will need dental treatment throughout our lives and it's important that when that happens people feel clear about the nature of the treatment and what it will cost upfront.

"We are calling on the NHS and the regulators to clean up dental costs and make sure the existing rules are put into practice consistently."

:: GMI surveyed 1,001 adults in England who had been to the dentist in the last six months online in November.

Barry Cockcroft, Chief Dental Officer, said: "Dentists have a duty to be open and honest about payment and treatment options, always considering the best interests of their patients."