Lack of access has prevented one in six people from seeing an NHS dentist for almost two years, a poll suggests.
NHS dentistry was reformed two years ago
Citizens Advice surveyed 1,800 people in England and Wales and found 300 had been frustrated - suggesting as many as 7.4m adults could be affected.
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.
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.
Citizens Advice chief executive David Harker said "People on low incomes are particularly affected as private treatment is just not an option."
The survey showed huge regional variations throughout the country, with the south west and north west of England the worst hit.
Lester Ellman, of the British Dental Association, said: "It is clear dentistry is not getting any better.
"The bottom line is that there are not enough dentists working in the NHS and that is because the government have failed to make it attractive enough."
He denied it was a matter of money and said instead dentists wanted to spend more time with their patients.
Health minister Ann Keen said: "We are working hard to improve access to NHS dentists and the government remains fully committed to expanding services.
"It is now a national priority for the health service."
She added the dentistry budget was being increased by 11% from this year to help achieve this.
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.
HAVE YOUR SAY
No NHS dentist for me. My dentist is in Budapest
Claire Jeffery, Sandy
New dentists have been gradually recruited to the health service, but figures are no higher than they were before the new contract.
Barry Cockcroft, Chief Dental Officer for England, said access to NHS dentistry had been a long-standing problem, which could not be ironed out straight away.
He said: "You won't turn it around absolutely completely in one year - but these reforms are a sound basis."
The Welsh Assembly Government said the new contract had been successful in developing dental services in Wales, with six new practices in recent months and others expanding the numbers of NHS patients.
A spokesperson said £30m additional funding had made a "noticeable difference" and problems of an access were now "confined to a very few areas".