A former Swiss private banker who was one of the first whistleblowers to use WikiLeaks by publishing internal bank documents on the site has pledged to hand over new data on offshore bank account holders on Monday, a newspaper said.

Rudolf Elmer, who was fired from Julius Baer in 2002 and who goes on trial in Switzerland on Wednesday for breaching bank secrecy, will hand over more data to WikiLeaks at a news conference in London, Der Sonntag reported on Sunday.

Elmer told the Swiss paper he would hand over two compact discs containing the names and account details of around 2,000 bank clients -- including prominent business people, artists and around 40 politicians -- who have parked their money offshore.

The documents show that they hide behind banking secrecy, probably to avoid tax, Elmer told the newspaper.

He said he understood the data would likely not immediately show up on WikiLeaks while it went through a vetting process, however.

He said the data involved multimillionaires, international companies and hedge funds from several countries including the United States, Germany and Britain.

The data came from at least three financial institutions, including Julius Baer, he told the paper, and covered the period from 1990 to 2009, with many of the documents leaked to him from other whistleblowers.

Neither Julius Baer nor Elmer, who was the bank's former chief operating officer in the Cayman Islands, were immediately available for comment.

After coming under heavy global pressure over the bank secrecy that allows foreigners to hide their assets from the taxman in Switzerland, Berne has agreed to do more to cooperate with other cash-strapped countries hunting tax evaders.

Switzerland agreed last year to transfer the details of around 4,450 clients who UBS AG helped dodge taxes to the U.S. government after it agreed to drop charges against the banking giant.

A former UBS banker-turned-whistleblower, who officials acknowledge was instrumental in the U.S. case against UBS, is now in jail on charges he helped a U.S. billionaire hide $200 million in assets.

Julius Baer has denied Elmer's allegations that it helped rich clients evade taxes and says he launched a campaign to seek to discredit the bank and some of its clients after he was dismissed. It also accuses Elmer of threatening individuals and altering documents.

Elmer, who has not been detained and can travel freely, first went to the U.S. Internal Revenue Service in 2007 with data on eight clients. He has also testified in a case in Germany related to an investment called Moonstone Trust.

He helped win global publicity for WikiLeaks when he first used it to publish information in 2007.

(Reporting by Emma Thomasson, Editing by Sonya Hepinstall)

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