Health
Lack of access has prevented one in six people from seeing an NHS dentist for almost two years.
Citizens Advice surveyed 1,800 people in England and Wales and found 300 had been frustrated.
It questioned people about treatment since April 2006 when a new NHS contract was brought in.
Health minister Ann Keen said the government was "working hard" to improve access to NHS dentistry.
Private treatment
Extrapolating from their poll result, the authors estimate that of the 7.4m who tried and failed to see an NHS dentist, 4.7m eventually opted for private treatment, and 2.7m went without treatment altogether.
Offical figures suggest the number of people who have failed to access NHS dentistry is 2m.
We are working hard to improve access to NHS dentists and the government remains fully committed to expanding services
The survey showed huge regional variations throughout the country, with the south west and north west of England the worst hit.
He denied it was a matter of money and said instead dentists wanted to spend more time with their patients.
She added the dentistry budget was being increased by 11% from this year to help achieve this.
Long-running problem
NHS dentistry has been a long-running problem for the government.
A new contract aimed at giving dentists more time with patients to get away from the so-called "drill and fill" culture was introduced as part of a reform of NHS dentistry in 2006.
It was hoped the move would stem the loss of NHS dentists to the private sector.
But despite getting paid the same for seeing less patients, 1,000 of the 21,000 NHS dentists in England refused to sign it, believing it was not as radical as they were led to believe it would be.