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The public spending watchdog is to investigate whether hundreds of thousands of people going to the dentist have been unfairly fined.

The National Audit Office is holding an inquiry into NHS penalty charges in England - applied to people alleged to be wrongly claiming free treatment.

The BBC had revealed many vulnerable people were being fined £100.

The British Dental Association (BDA) has warned that this is scaring poor families from going to the dentist.

The dentists' organisation says almost 430,000 penalty fines were issued last year - which they say is often "simply for ticking the wrong box on claim forms".

'Low incomes and elderly'

Many of the people facing fines, the dentists say, are "on very low incomes, the elderly, and those with learning difficulties".

The BDA says that when these fines for wrongfully claiming free treatment are challenged, about nine in 10 are overturned.

But dentists are worried that many people are so worried about the risk of being fined that they do not get dental treatment.

Image caption Dentist Charlotte Waite says too many vulnerable people are being unfairly fined

The BBC highlighted the concerns of many families who found that elderly relatives, or people with dementia or learning difficulties, were getting fined, even though they were entitled to free treatment.

The growing scale of the problem has emerged - and also the impact on poorer patients.

According to the BDA, the number of dental visits from those entitled to free treatment has fallen by almost a quarter in the past four years.

Compared with five years ago, there has been a tenfold increase in the number of fines.

'Hostile environment'

The National Audit Office is to examine how the NHS penalty charge system is being operated - how many are issued, entitlement to exemptions and the revenue generated from fines.

The watchdog, expected to report during the spring, will examine what the NHS Business Services Authority, the Department of Health and Social Care and NHS England, are doing "to improve the efficiency of the system".

Charlotte Waite, the BDA's chair of the community dental services committee, welcomed the investigation - and said a simple error in an application form should not mean a £100 fine.

"The government's approach to penalty charges has hit hundreds of thousands of vulnerable patients, and encouraged millions more to miss out on care.

"Yes, we need a system to protect taxpayers' money, but that does not mean constructing a hostile environment for patients, many of whom have complex needs," said Ms Waite.

The Department of Health and Social Care has argued that everyone should have free NHS dental treatment if they are eligible.

But it has said that it is right for it to seek to recover money from anyone who wrongly claims free services.