Nearly two-thirds of dentists in England are turning away NHS patients, according to a survey.
In 2005 69% of dentists were turning patients away
Which? said the response from 64% of the 466 dental practices it polled was only 5% down on the figure before a new NHS contract started in April 2006.
The consumer group said ministers were not giving local health bosses enough money to pay dentists for services.
But Health Minister Rosie Winterton said reforms were improving access to dentistry across the country.
The government introduced a new deal last April giving dentists the same money for treating fewer patients in a bid to get away from the "drill and fill" culture.
Primary care trusts, which have been given the responsibility for dental care, were told a quarter of their budget had to be made up from fee-paying patients - most adults contribute to the cost of their NHS treatment.
But many PCTs have reported that they are unlikely to take quite as much as expected in fees, meaning they have not been able to fund dentists to treat more patients.
Which? found that just 36% of dental practices were taking on new patients - up from 31% in 2005.
And they also found there were variations across the country. Some 63% of West Midlands dentists and 59% of London ones were taking on new patients, compared with just 13% in the north west.
Separate figures from the NHS Information Centre show that 20,887 dentists are now working in the NHS - up by 602 in the past three months.
But this is less than the 21,111 employed before the new contract started. When the deal came in about 1,000 dentists left the health service, either to retire or concentrate solely on private work.
Frances Blunden, health campaigner at Which?, said: "Where needs are not currently being met by the NHS, people are either putting off having treatment or are being forced to go private.
"If the government is serious about creating a patient-centred approach to NHS dentistry, then PCT allocations must be related to local needs."
Lester Ellman, of the British Dental Association, said: "The figures reflect what we have been hearing. This new contract was meant to sort problems, but it was never popular with dentists in the first place and now it seems the government got its sums wrong.
"The people who suffer the most are the ones on low incomes who cannot afford to go private and these are the people most in need."
Lib Dem health spokeswoman Sandra Gidley said: "It is totally unacceptable that so many patients do not have access to an NHS dentist."
Andrew Lansley, shadow health secretary, added: "The new contract is not serving patients or dentists well. NHS dentistry is costing the NHS considerably more; and yet in many parts of the country patients are finding that they cannot get access to it."
Health Minister Rosie Winterton said the old system had been "deeply flawed" with "gaps in access".
She added: "Thanks to the reforms, PCTs now have the power and money to improve and develop local dental services, and are now turning the corner in improving access to dentistry."