Almost a third (31%) of dentists listed as accepting new NHS patients by NHS Choices are not, research by Which? has found.
Of those surgeries able to offer appointments, 29% couldn't offer an appointment within two weeks, and one saying there was a wait of eight to nine months.
Follow up research at the start of the fiscal year 2015/16 found that 37% of surgeries wouldn't give an NHS appointment and 36% couldn't offer an appointment within a fortnight.
Some practices asked for deposits from new patients, and in one case the Which? researchers were told the deposit was non-refundable if the patient missed the appointment, practices not permitted by legislation.
Other practices asked patients to visit the surgery to fill in forms before giving appointments while some practices suggested paying privately when specifically asked for an NHS appointment.
Which? is calling on the Competition and Markets Authority to ensure dentists comply with existing rules and improves the way treatment options, prices and appointment availability are communicated to patients.
The "Clean Up Dental Costs" campaign was launched by Which? in January 2015.
Which? researchers called 500 dentists, all listed on the NHS Choices website to book an appointment.
Richard Lloyd, executive director at Which?, 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."