Standards of NHS dental care in England have been likened to a "Third World" service, by dentists.
The care, already "unfit for purpose", is becoming even worse, a letter signed by 400 dentists, in the Daily Telegraph newspaper, adds.
It says the creation by international charity Dentaid of a service for vulnerable patients in West Yorkshire is a sign of the mounting problems.
NHS England said services were improving.
An NHS England spokeswoman said: "These claims are wrong - more patients are getting the dental care they need, and 93% of people got an NHS dental appointment when they wanted one in the last 24 months."
The letter urges ministers to be "fully open and transparent" about the limitations of current provision.
The fact Dentaid, normally associated with providing care in the Third World, is now offering emergency care to homeless people, migrants and the low paid in Kirklees "serves to demonstrate the lack of a proper national dental service", it says.
The letter also calls the levels of tooth decay in children a "national disgrace".
Figures published last year by the Health and Social Care Information Centre showed nearly half of eight-year-olds and a third of five-year-olds had signs of decay in their milk teeth.
The Children's Dental Health Survey for England, Wales and Northern Ireland, carried out every 10 years, also found 46% of 15-year-olds had decay in their teeth - although that represented an improvement on the previous survey.