A Which magazine undercover investigation of 25 dental practices has rated a significant proportion as either "poor" or "very poor" at explaining prices and treatment options.
An "expert panel" assessing hidden recordings rated eight of the 25 practices as poor or very poor at explaining their prices with the same results when it comes to explaining treatment options.
The practices were found to be weakest at explaining NHS versus private treatment options with 12 rated as poor or very poor.
Which claims its investigators found a "muddled middle" where the difference between NHS and private options needs to be clearer. On four of the undercover visits, researchers were offered private hygienist appointments when they should have been offered an NHS scale and polish and advice on looking after their teeth and gums.
Only half of the undercover researchers saw a price list prominently displayed, with the majority of practices not showing any private prices.
The Which investigators also said: "It is a legal requirement for patients offered band 2 or 3 NHS dental treatment - such as fillings or root canal treatment - to be given a written treatment plan. On our undercover visits, fewer than half (eight out of 20) of those eligible were given a treatment plan after their visit to the dentist."
Which has launched a Clean Up Dental Costs campaign calling on NHS England and regulators to make sure all dentists comply with existing rules and make information on prices clearly available, explain dental treatment options properly and make sure patients know whether or not their treatment is available on the NHS.
Executive director of Which, Richard Lloyd, said: "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."
Mick Armstrong, Chair of the British Dental Association commented: "It is essential that patients have a clear understanding about treatment options and costs when they visit their dentist. Unfortunately the rules determined by government have proved a recipe for confusion. Neither the NHS contract nor what the NHS will pay for is clear enough. It's a system that is failing patients and practitioners alike.
"The BDA supports recommendations about clear published price lists as this helps to ensure both the patient and the dentist share a common understanding. NHS posters are not exactly eye-catching, so we are not really surprised that a minority of patients don't remember seeing them long after their visit - but that doesn't necessarily mean that they weren't there.
"Indeed, the CQC investigates pricing information as part of its inspection process. Nearly all 10,000 practices in England have been inspected by the CQC and 95 per cent have met their overall inspection standard."