An Ammanford doctors' surgery is at the centre of a massive fraud probe.

The Guardian has learned that the London-based NHS Counter Fraud and Security team has been called in to investigate the alleged disappearance of a six-figure sum from the Margaret Street Practice.

Workers at the surgery have been forced to change locks to the practice doors after it was alleged that a current or former member of staff was behind the scam.

When contacted over the incident, a spokesman for the Margaret Street Practice refused to confirm or deny any details of the alleged deception.

"I have no comment to make whatsoever," said the spokesman.

NHS officials were also reluctant to offer any insight into the alleged incident, but refused to rule out that it had taken place.

"I cannot confirm anything at all at this moment in time," said Matthew Willis, a spokesman for the NHS Counter Fraud and Security management team.

"All I can say is that the NHS counter fraud service investigates all allegations of fraud within the NHS thoroughly and properly."

The Carmarthenshire Local Health Board also said its officers would have no involvement in the investigation process.

"This is an issue for the surgery itself," said a spokesman.

"Although the LHB commissions surgeries such as Margaret Street, they are actually run as individual businesses in their own right.

"It is the practice itself which would investigate such cases, rather than the LHB.

"As the person involved has been employed by the surgery and not the LHB, the incident would be investigated by the surgery.

"This is an employment issue and we do not deal with employment issues."

The Margaret Street Practice came into being in August 1986 with the construction of a purpose-built surgery following a series of mergers between practices at Ammanford's Hall St, Llandybie and Tycroes.