Spanish police have arrested nine people accused of stealing from customers at two firms that specialise in investing in postage stamps.
Both firms have had their activities frozen by the police
The staff at Forum Filatelico and Afinsa Bienes Tangibles are suspected of fraudulently taking a "substantial" amount of money from 250,000 investors.
Police also suspect that some of the stamps are "overvalued - or even fake".
Afinsa, the world's biggest stamp dealer, insisted all the stamps it bought and sold were real.
It added that it was working "totally normally" to deliver on its commitment to its clients.
Forum Filatelico was not immediately available for comment.
Spanish police said on Wednesday that they did not know exactly how much money was involved.
"Potential investors were offered high returns from the purchase and management of a stamp fund, which was apparently made up of overvalued - or even fake - stamps and whose returns did not apparently come [from the fund] but from money received from new clients," said Spanish police in a statement.
Spanish police have now sealed the safes of the two companies and raided 21 homes and offices in total.
They have also frozen all business activities at both firms.
As Afinsa and Forum Filatelico are not strictly financial companies, they are not covered by Spanish government guarantees.
Government spokesman Fernando Moraleda said Madrid would defend the rights of investors affected by the possible frauds, which dated from 1998 to 2001.
Afinsa has a 67% stake in US auction house Escala.
Escala's shares fell as much as 58% in Tuesday trading in New York.