The Serious Fraud Office is already investigating what could be the first big UK frauds to come to light in the current financial crisis.

The SFO, Britain's major fraud investigation unit, revealed the existence of "at least one" serious probe, and called on City workers to blow the whistle on chicanery as the financial system reels from the Bernard Madoff scandal.

The SFO, which has been the subject of much criticism in the past for failing to nail fraudsters, today launched a website for people to rat on their employers or colleagues.

The unprecedented initiative comes after new SFO chief Richard Alderman visited counterparts in the US last month. The SFO said it is "posting a special form on its website so that those employed in the City's financial services industry can blow the whistle on any bad practices.

"It is targeting individual workers, ex-employees, shareholders and anyone with information about suspected City fraud."

An SFO insider said an internal shake-up to make the SFO more receptive and flexible to tip-offs has already led to the launch of at least one investigation, into corporate "discrepancies" that could amount to a serious fraud.

Alderman has launched the whistleblowing initiative after meeting up with officials at Federal Bureau of Investigation, the Securities and Exchange Commission, the Department of Justice and the New York district attorney's office in November.

"We've found in the past that at times of market turbulence fraud is discovered," he said.

Andrew Gordon, head of forensic investigations at accountants PricewaterhouseCoopers, said: "The SFO is absolutely right in its timing as we are coming into the end-of-the-financial-year reporting season when things begin to appear. Statistics show that 50% of the SEC's investigations have come through people contacting their hotline."