Dutch authorities have given hundreds of Credit Suisse clients an ultimatum, in a six-country investigation into alleged tax evasion using hidden Swiss accounts.

Holland's anti-fraud authority Fiscale Inlichtingen- en Opsporingsdienst, or FIOD, sent a letter to 650 clients of Zurich-based Credit Suisse this week. In the letter, the authority urged clients to come clean about any money they may have secreted away in the Swiss bank, Dutch paper «Het Financieele Dagblad» (in Dutch) reported.

Dutch authorities shocked the bank last month by raiding its office in Amsterdam, and coordinating another raid in Paris and visits to offices in Germany and London. Investigators in the Netherlands also confiscated luxury cars, property and a gold ingot in raids in four Dutch cities, while a British woman was arrested earlier this month.

Authorities, who were helped by American officials providing data, have apparently received information about 3,800 Dutch clients with accounts at the Swiss bank.

Jump in Declarations

Credit Suisse maintains that it swept its accounts clean of tax cheats; its chairman Urs Rohner, who was chief lawyer and operating chief until 2009, said on Sunday that the bank still doesn't know the nature of any allegations against it.

Since the first raid nearly three weeks ago, a carefully-coordinated plan by tax authorities in Holland, France, Germany, Britain, Australia and Austria has emerged.

Dutch authorities now suspect «dozens» of citizens of having stowed millions of euros with «a Swiss bank,» the newspaper reported. Clients still hiding untaxed money face penalties of 120 percent of the undeclared funds – wiping out the principal and then some – if authorities find them before they come clean.

So-called voluntary disclosures, or clients coming clean on their hidden funds, have jumped dramatically since news of the raids broke, the paper reported.