Some newly-qualified dentists are being prevented from treating NHS patients because of delays in issuing paperwork, says the British Dental Association.
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To be allowed to work for the NHS, dentists need a "performer number".
The NHS used to deal with applications, but they are now handled by Capita and instead of an average six weeks, can now take up to seven months to process.
In a statement, Capita said the "timely processing" of the numbers was a "key priority" for the company.
The BDA think there could be as many as 300 dentists in this position, although there is nothing to stop them from working privately.
Henrik Overgaard-Nielsen, chairman of the general dental practice committee at the British Dental Association, said: "This is a clear case where a private company has taken over a public sector contract and is failing to deliver.
"Thousands of patients are losing out and hundreds of dentists cannot earn their living."
"The patients are suffering because of even longer wait times and we can't earn a living."
Last year a grace period which allows dentists trained within England to operate on the NHS without a performer number was extended from 30 November 2016 to the end of January this year, but this does not apply to those who qualified elsewhere.
This was to allow Capita more time to issue the numbers, but the BDA says many have still not been received.
Capita, operating under the banner of Primary Care Support England, took over running some back office operations from the NHS in April 2016.
Since then they have cut the number of offices processing NHS administration from more than 40 to only three.
Capita said: "The robust and timely processing of performer list applications remains a key priority for us and we are currently making enhancements to this process."
NHS England acknowledged there was a problem, saying in a statement: "NHS England has provided a team of experts to support Capita's work, both in managing existing work more effectively and improving the service for the future."