Switzerland's top criminal court yesterday granted Spain access to secret Swiss banking details in an investigation into a suspected stamp collecting scam that defrauded around 350,000 people, according to AFP.

Spanish media sources have reported that investigators believe two major stamp trading companies laundered €3.5 billion (around $4.7 billion) obtained from fake sales of stamps through foreign bank accounts, including in Switzerland.

The Swiss Federal Criminal Tribunal rejected an appeal by the bank account holders, whose names were not made public, seeking to overturn an earlier judgement accepting Spain's 2009 request for legal assistance in the matter.

The case throws light on the steps countries must take to probe into secret Swiss bank accounts, which have been under pressure in recent years due to claims that foreign nationals are secreting funds in the Alpine state to evade tax.

The AFP report said the court ruling noted that magistrates in Madrid suspected one of the appellants of being behind the "falsification of official stamps" and jointly involved in a "huge philatelic fraud."

Spanish savers were attracted by the promise of market beating profits from stamps held in deposit, according to court documents. But magistrates in Spain believe the stamps never existed and the savers were defrauded in a giant pyramid scheme that was fuelled by the flow of fresh cash from unsuspecting new investors.

A magistrate in Geneva froze the suspect Swiss bank accounts last year.