Data from England and Wales shows more than 2,500 dental posts were lost across both countries – made up of more than 1,000 dentists, some of whom worked in multiple areas.
For the West Sussex Clinical Commissioning Group (CCG) numbers rose from 565 in 2017 to 595 in 2018 but have have since been on the downturn with 593 in 2019, 547 in 2020 and 527 in 2021.
The area has lost 38 dentists in the past five years.
The British Dental Association (BDA) said unhappiness with the NHS dental contract was a key factor.
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.
NHS England said patients who needed care the most should be prioritised, and said it had set up 600 urgent dental centres across England.
An NHS spokesperson said: “The NHS has taken unprecedented action to support NHS dentists throughout the pandemic by providing additional funding for practices unable to deliver their usual levels of activity, alongside rapidly setting up 600 urgent dental centers across England so patient services could be maintained during the pandemic.
“People should continue to come forward for the dental care they need, and the care and treatment of people who need it most should be prioritised.”
The worst-affected was NHS Portsmouth CCG, which lost 26 per cent of its NHS dentists over 12 months.
Meanwhile, 28 other English CCGs have lost at least 10 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.”
Data supplied by BBC Shared Data Unit and BBC Local News partnerships