The British Dental Association says a "huge fall" in people on low incomes going to the dentist in England is because they fear being wrongly fined when they claim for free treatment.
The most recent annual figures show almost 370,000 fines for people accused of "misclaiming" for free dental care.
But dentists say many of these are innocent confusions over paperwork – with 90% overturned when challenged.
The NHS says that anyone entitled to free care should be able to access it.
Those claiming for free treatment include the elderly, those on benefits and people with learning difficulties.
And the BDA says the system of fines, usually of £100, is creating a "hostile environment" for the vulnerable.
Dentists have warned that simple errors in filling in a form can lead to fines being issued – and they say that these are making people afraid to seek dental care, even though they are exempt from charges.
The BDA says that this is driving a downturn in visits to the dentist by low-income people, falling by 23% over four years. This represents two million fewer treatments in 2017-18 compared with 2013-14.
The number of fines increased more than tenfold across four years – from almost 34,000 to 365,000. The BDA says that when these fines are examined in appeals, a large majority of them are overturned.
Dentists' leaders say they have "grave concerns that patients are being put off from seeking treatment by the government's aggressive approach to fines".
"This huge fall in NHS attendance amongst patients exempt from NHS charges is the logical outcome of failed government policy," the BDA's chairman of general dental practice, Henrik Overgaard-Nielsen, said.
"Ministers have created a hostile environment for vulnerable groups and those on low incomes who have a right to free dentistry.
"These patients, often with complex needs, require early intervention not the ever-present threat of fines," he said.
Last year, dentists and patients contacted the BBC to warn of how the fining system was becoming a barrier to dental care.
There were complaints from distressed families that vulnerable people, such as those with dementia, cancer patients and people with multiple disabilities, could face fines as a result of simple errors or confusion over forms.
Charlotte Waite, a senior dentist working in Loughborough, Leicestershire, told the BBC this was a problem appearing on a "daily basis".
The fines for wrongful claims are intended to stop fraudsters and to make sure no-one who should pay for treatment is avoiding charges.
But the NHS had recognised that there were concerns about the system and had emphasised the importance of providing accessible information for patients to make sure they knew whether they were entitled to free treatment.