The papers were handed to The Independent on Sunday, which said that they covered the UK's policies on fighting global terrorist funding, drugs trafficking and money laundering.

They were said to include details of how trade and banking systems could be manipulated to finance illicit weapons of mass destruction in Iran.

They also discussed methods of terrorist funding and the potential fraud of commercial websites and international internet payment systems.

The confidential files - which were discovered on a train bound for London's Waterloo Station on Wednesday - related to a week-long conference of the international Financial Action Task Force (FATF).

They include speaking notes for a senior official at a reception to be held at 11 Downing Street, Chancellor Alistair Darling's official residence.

The papers were found on the same day that a top secret assessments by the Joint Intelligence Committee covering Iraq and al Qaida were handed to the BBC having also been left on a train.

Shadow security minister Baroness Neville-Jones said the Government must "get a grip" on the issue of data protection.

"This is another incidence of the failure of the Government to safeguard sensitive information and yet another example of a lapse in discipline," she said.

The Independent on Sunday said it had returned the documents and would not be divulging any details contained in them.