Rudolf Elmer, who was sacked 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, the news service quoted Der Sonntag as saying.

This is not the first time that Wikileaks has published details of banking clients. The loan books of Kaupthing, which had a UK operation that has been now liquidated amid the financial turmoil, were published on the Wikileaks site in 2009.

The “whistleblowing” website has published details from governments and private sector organisations, creating a storm of controversy about whether its activities are in the public interest or if they damage security – such as in Afghanistan - and compromise client privacy. A number of private banks, such as LGT in Liechtenstein, have seen former employees steal client data. The German government, for example, has paid public funds to obtain such information, raising questions about respect for due process of law and the extent to which governments are prepared to go after alleged tax dodgers.

In the latest report on Julius Baer, Elmer said 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.

"The documents show that they hide behind banking secrecy, probably to avoid tax," Elmer told the newspaper. Tax avoidance and evasion are not crimes in Switzerland, although evasion is treated as a crime in countries such as the US and UK and, increasingly, governments are also seeking to crack down on avoidance.

He reportedly said the data involved multimillionaires, international companies and hedge funds from several countries including the US, 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 sources.

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

Elmer was involved in leaking documents to Wikileaks. The website, against whom the Swiss bank dropped a case in 2008, is not party to the trial this week. Elmer will answer charges of coercion and violating Switzerland's banking secrecy laws on 19 January, with the bank acting as a plaintiff.

The trial will instead focus on the earlier release of data to Swiss media, the bank has told WealthBriefing.

Back in February 2008, Wikileaks was ordered by a California court to shut down, at the request of Julius Baer and its Cayman-based subsidiary Julius Baer Bank & Trust. This was following a court case, initiated because the website had posted documents relating to the offshore activities of Julius Baer. The following month, the Swiss bank voluntarily dropped the case against the whistle-blowing website.

The leaked documents were based on data allegedly supplied by Elmer, a former chief operating officer of Julius Baer in the Cayman Islands. The bank, however, denied the authenticity of the material at the time, and rejected the allegations.

In the upcoming case, which is entirely separate from the previous one, Elmer has said he will admit some counts of coercion but not to breaking Swiss banking secrecy, because the files he distributed belonged to a Cayman subsidiary of the bank, Bloomberg has reported.