The decrease has been steady over the last five years, with 202 NHS dentists in the region in 2017, 189 in 2018, 179 in 2019, 166 in 2020 and 162 in 2021.
Along with East Riding of Yorkshire CCG also on 20 per cent, this is one of the biggest five-year decreases in England, with only Salford (28 per cent) and Portsmouth (25 per cent) higher.
The figures come as unions warn NHS dentistry is "hanging by a thread" with some patients facing two-year waits for routine check ups.
Data from NHS England and NHS Wales shows more than 2,500 dentists - up to eight per cent of the workforce - stopped treating NHS patients last year.
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 in two English clinical commissioning group areas (CCGs) fell by more than a quarter in the year ending March 31 2021.
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 Simon 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 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 dentists stopped treating NHS patients, with 83 fewer dentists doing NHS work than the year before.
The worst-affected area was the Swansea Bay University Health Board, which had 22 per cent fewer dentists compared to the year before.
Until last year, NHS dentists in England and Wales had been using the units of dental activity (UDA) system.
UDAs are used to measure a practice’s activity. Courses of treatment - for example, a check up or a filling, are banded into UDAs.
Practices are set targets of UDAs to achieve, and if that target is missed, the contract holder and the practice can be forced to pay back money - known as ‘claw back’.
Critics have claimed the UDA system does not incentivise preventative work, and is a key reason for dentists leaving the health service.
Wales moved away from the UDA system in 2020, but the system is still in place in England.
The BDA has predicted the number of NHS dentists in Wales will increase in the coming years and has called on England to reform the dental contract.
The number of NHS and HSCNI dentists in Scotland and Northern Ireland has remained steady over the last three years.
An NHS spokesman 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.”