The Serious Fraud Office and Ministry of Defence police, which handle Royal Mint security, are to investigate claims over payments made between 1996 and 1999.

Concern over the deals was raised by Gerald Sheehan, appointed Mint's deputy master last December.

Results of an internal investigation were received by the Treasury in June.

Overseas contracts

There are believed to be fewer than 10 payments, amounting to thousands of pounds involved.

There are also unconfirmed claims of "bribe" payments involving overseas contracts, the Financial Times said.

The Mint, which traces its history back for 1,000 years, has sought in recent years to bolster its overseas client base.

"[The Mint's] reputation for the world's finest coins means that more than 100 countries have entrusted the striking of their coinage to the Royal Mint," the organisation claims.

While plans to part-privatise the Mint were shelved three years ago, the firm has been allowed greater freedom in its business operations.