A third of dentists who claim they are accepting new patients do not, while many that do leave their patients with lengthy waits, consumer watchdog Which? has found.
Its team of undercover researchers found that of the 500 dental surgeries advertised as accepting new patients on NHS Choices, 37% said they did not have availability, while of those that could offer an appointment, 36% said they would have to wait two weeks.
One surgery said it would be eight to nine months before an appointment would be available.
Which? said it was asking the Competition and Markets Authority to step in over the problem, which it said was part of a wider issue of poor information and communication in dentistry.
Its investigation found that some researchers were asked to pay deposits for NHS appointments, and in one case were told this was non-refundable if the appointment was missed - even though neither of these practices is permitted under Government legislation.
Which? said the issues were previously identified by the Office of Fair Trading in its dentistry report in 2012, yet three years on the sector has failed to deliver on the recommendations made.
Which? executive director, Richard Lloyd, said: "We found it's frustratingly difficult to get an NHS appointment with a dentist as information about availability doesn't reflect reality. This is a kick in the teeth for patients and yet more evidence of poor communication from the dental sector.
"We want the Competition and Markets Authority to step in and ensure that dentists put the existing rules into practice so that people can easily find out where they can get NHS dentistry."
The British Dental Association (BDA) said a "comprehensively flawed payments system at the heart of NHS dentistry" was a big factor.
Since April 2006, NHS dentists in England and Wales have been paid according to how many Units of Dental Activity (UDA) they do in a year, rewarding dentists for completed treatment on the basis of a "points" system, which the BDA said has fuelled waiting lists.
BDA chairman Mick Armstrong said: "The Government remains committed to a byzantine system that has failed both dentists and their patients.
"From day one these arbitrary targets have proved a real obstacle for new NHS patients. Dentists often have more patients lining up than they have UDAs, but the UDA must come first.
"Many would like to see more patients, but this is impossible within rigid contracts.
"For far too long oral health has been left out of the health debate, and this new evidence provides fresh impetus for Government to reassess its agenda."