Liberal Democrat health spokesman Steve Webb has warned NHS dentistry services are "on the brink of disaster".
Contracts 'may hit preventative work'
He said many dentists were threatening to quit the NHS because of new contracts which pay on the basis of the number of treatments performed.
Mr Webb said less than half of people in England were currently registered with an NHS dentist and predicted the contracts would see that figure fall.
The Lib Dems are due to debate the issue at their conference in Blackpool.
Mr Webb will be speaking for a motion calling for action to encourage dentists to register more NHS patients.
Earlier Mr Webb visited a surgery in the seaside town, where he said: "This is a critical point for NHS dentistry. There has been a long-term decline with the proportion of people - adults and children - registered with a dentist falling for the best part of 15 years.
"We are getting a lot of feedback from dentists saying that the new contract is the last straw and that they are thinking of closing their doors to NHS patients."
The new contracts, being introduced next April, pay on the basis of treatments performed.
This means that dentists will be under pressure to see ever more patients to maintain their income, and so have little time for preventative work, said Mr Webb.
"The worry is that children in particular will not see a dentist and will build up a lifetime's problems with their teeth. They will end up in dental hospitals and casualty units, which will be much more expensive for the NHS," he said.