The overseas bank account details of 2000 "high-net worth" individuals and corporations - detailing widespread possible tax evasion - will be handed to WikiLeaks tomorrow.
The organisation is receiving the details from the most important and boldest whistleblower in Swiss banking history, Rudolf Elmer, two days before he goes on trial in his native Switzerland.
British and American individuals and companies are among those whose details are on CDs to be presented to WikiLeaks in London.
They include about 40 politicians, Elmer says.
Elmer, who after his press conference will return to Switzerland from exile in Mauritius to face trial, is a former chief operating officer in the Cayman Islands for the Julius Baer bank, which accuses him of stealing the information.
He is also - at a time when the activities of banks are a matter of public concern - one of a small band of employees and executives seeking to blow the whistle on what they see as unprofessional, immoral and even potentially criminal activity by powerful international financial institutions.
Switzerland is a fortress of banking and financial services, but is famously secretive and expert in concealing wealth from all over the world for tax evasion and other extra-legal purposes.
Elmer says he is revealing the information " to educate society".
He says his list includes "high-net worth individuals, multinational conglomerates and financial institutions - hedge funds".
They are said to be "using secrecy as a screen to hide behind in order to avoid paying tax".
They come from the US, Britain, Germany, Austria and Asia.
Clients include "business people, politicians, people who have made their living in the arts and multinational conglomerates - from both sides of the Atlantic".
Elmer says: "Well-known pillars of society will hold investment portfolios and may include houses, trading companies, artwork, yachts, jewellery, horses, and so on.
"What I am objecting to is not one particular bank, but a system of structures," he said.
"I have worked for major banks other than Julius Baer, and the one thing on which I am absolutely clear is that the banks know, and the big boys know, that money is being secreted away for tax-evasion purposes, and other things such as money-laundering."
Elmer was held in custody for 30 days in 2005, and is charged with breaking Swiss bank secrecy laws, forging documents and sending threatening messages to two Baer officials.
Elmer says: "I agree with privacy in banking for the person in the street, and legitimate activity, but in these instances privacy is being abused so that big people can get big banking organisations to service them. The normal, hard-working taxpayer is being abused also."
The names on the CDs will not be made public, just as a list of 15 clients that Elmer gave WikiLeaks in 2008 has remained undisclosed.