At least one town in England has been unable to attract a single applicant for vacant NHS dentist posts for two years.
The British Dental Association (BDA) said unhappiness with the NHS dental contract was a key factor.
NHS England said patients who needed care the most should be prioritised, and said it had set up 600 urgent dental centres across England.
The number of NHS dentists working in two English clinical commissioning group areas (CCGs) fell by more than a quarter in the year ending March 31, 2021, with the combined equivalent of 2,435 dentists leaving the health service.
The worst-affected was NHS Portsmouth CCG, which lost 26 per cent of its NHS dentists over 12 months.
In Doncaster there were 192 NHS dentists which fell to 178, that’s seven per cent in total.
Meanwhile, 28 other English CCGs have lost at least ten per cent of their NHS dentists.
The BDA’s Shawn Charlwood warned significant numbers of dentists were planning on leaving the NHS.
“NHS dentistry is hanging by a thread, because without NHS dentists, there will be no NHS dentistry,” said Mr Charlwood.
“It’s a really serious situation and every dentist that is lost or every vacancy for NHS dentistry that remains unfilled affects thousands of patients in terms of care and their ability to access care.”
The BBC understands that one dental practice in Barnsley has had two NHS dental posts vacant for two years - without attracting a single applicant.
"Every practice struggling to fill vacancies translates into thousands of patients unable to access care,” said Mr Charlwood.
"Years of failed contracts and underfunding have meant a growing number of dentists no longer see the NHS as a place to build a career. The pandemic has upped the ante, and we are now facing down an exodus.
"Ministers have failed to grasp that we can't have NHS dentistry without NHS dentists.
“Rather than punishing colleagues, we need a service that recognises and rewards commitment.”
Concern has also been raised about the usefulness of NHS England’s ‘Find a Dentist’ tool, which was created to help patients find an NHS dentist in their area.
BBC analysis shows around 75 per cent of practices in England had not updated the site to show whether they were accepting NHS patients or not within the last three months.
Interim director of Healthwatch Chris McCann said getting up to date information as to where people can access service is a “real issue”.
“Information on practices on the NHS website can be out-dated,” he added. “We've seen some people having to contact up to 20 practices before finding someone to take them.”
In Wales, six per cent of NHS dental posts were lost, with 83 fewer dentists working across health boards than the year before.
The worst-affected area was the Swansea Bay University Health Board, which had 22 per cent fewer NHS dentists compared to the year before.