Patients have had urgent procedures cancelled at the last minute and been told to wait for months or find another dentist.
Others with painful tooth infections have been prescribed antibiotics, contrary to NHS guidelines, because no dentists are available to undertake treatment.
Mydentist, which has ten million patients on its books, admits it does not have enough dentists to carry out check-ups and treatments
Mydentist, which has 650 practices in the UK, is struggling to fill posts due to a national shortage of dentists. Many are abandoning their NHS patients to concentrate on private and earn more money.
Others are moving abroad because they have had enough of strict rules which limit the numbers of check-ups and procedures they can offer to patients each year.
But patients of Mydentist clinics have taken to internet forums to complain of appointments being cancelled, procedures suspended and, in some cases, treatment refused altogether.
The company, managed by the Carlyle Group based in Washington DC, received £385.1million of NHS funding in 2017.
One patient who attended a Mydentist clinic in Brighton, East Sussex, said their root canal treatment was suspended between appointments and they were advised it would not be completed for five months.
He said: 'Mydentist informed me that they couldn't do anything. They were unable to offer me any further advice. I had to find another dentist.'
Others with painful tooth infections have been prescribed antibiotics, contrary to NHS guidelines, because no dentists are available to undertake treatment
Another patient at a clinic in Hayes, West London, said: 'I feel I would have better treatment if I was a cat and booked an appointment with a vet.' A patient in Plymouth said they had five appointments cancelled without warning in just six months.
The British Dental Association, which represents dentists, said the problems are caused by Government cuts to dental services and overly restrictive targets. Government spending on NHS dental services has dropped from an average of £41 per head to £36, and that has hit dentists' salaries.
A dentist treating NHS and private patients can typically earn £75,000 to £100,000 a year. Those who treat solely private patients usually earn more than £140,000.
Many dentists are also unhappy with their contracts which dictate they can only carry out a certain number of check-ups and treatments each year. If they go over that limit, they won't be reimbursed by the Government so they have to make patients wait for the next year. Growing numbers are moving abroad or switching to private work only.
A Department of Health and Social Care spokesman said: 'We are committed to supporting dentists and making sure the public can access top quality care, which is why we are trialling a reformed contract which better supports dentists, and have near record numbers of dentists working in the NHS. NHS England monitors local recruitment closely and we expect them to take appropriate action where any issues are identified.'