A former Swiss banker has passed on data containing account details of 2,000 prominent people to Wikileaks founder Julian Assange.
The data - which is not yet available on the Wikileaks website - was held on two discs handed over by Rudolf Elmer at a press conference in London.
Mr Assange promised full disclosure once the information had been vetted.
Mr Elmer is scheduled to go on trial in Switzerland on Wednesday for breaking bank secrecy laws.
The banker, who has given data to Wikileaks before, was fired from Swiss bank Julius Baer in 2002.
"Evidently disgruntled and frustrated about unfulfilled career aspirations, Mr. Elmer exhibited behaviour that was detrimental and unacceptable for the Bank, which led to termination of the employment relationship," the bank said in a statement sent to BBC News.
"After his demands (including financial compensation) in connection with the dismissal could not be satisfied, Mr. Elmer embarked in 2004 on a personal intimidation campaign and vendetta against Julius Baer," the statement read.
Serious Fraud Office
Although it was not confirmed what activities might be covered by the data Mr Elmer has passed on, the Wikileaks head noted that previous data from Julius Baer provided by Mr Elmer had shed light on tax evasion, the hiding of proceeds of criminal acts and "the protection of assets of those about to fall out of political favour".
The data covers multinationals, financial firms and wealthy individuals from many countries, including the UK, US and Germany, and covers the period 1990-2009, according to a report in Swiss newspaper Der Sonntag.
"Once we have looked at the data... there will be full revelation," said Mr Assange, who is currently on bail and confined to the UK due to an extradition request from Sweden.
The Wikileaks founder has been accused of sexual misconduct by two women in Sweden, including having unprotected sex without consent - accusations he denies.
Speaking at the handover event at the Frontline Club, he said the data would be vetted before publication.
It was difficult to say how long this would take, he said, although he suggested it could be as little as two weeks.
The vetting would depend on the volume of information and how it was delegated, Mr Assange said.
Other groups - such as the Tax Justice Network or financial media outlets - might be asked to help in the vetting process, he added.
Mr Assange also said some information was likely to be handed over to the authorities - mentioning specifically the UK's Serious Fraud Office - as was the case with a previous leak concerning Icelandic banks.
Julius Baer has dismissed Mr Elmer's accusations.
"The aim of his activities was and is to discredit Julius Baer as well as clients in the eyes of the public," it said in the statement.
"With this goal in mind, Mr Elmer spread baseless accusations and passed on unlawfully acquired respectively retained documents to the media, and later also to Wikileaks.
"To back up his campaign, he also used falsified documents and made death threats against employees."
The banker's latest revelations also relate to other Swiss banks.
A spokesman of the Swiss Bankers' Association (SBA) told the BBC his country was "a sophisticated, well-regulated international financial centre with some of the strictest know-your-customer rules worldwide".
Jean-Marc Felix - an SBA board member - insisted that the country complied with international standards on tax matters, and the banking community supported moves to set up bilateral agreements with the UK and Germany to introduce a withholding tax on accounts of citizens from those countries.
"I'm against the system. I know how the system works," said Mr Elmer at the press conference. He said a sophisticated network existed to funnel illicit money into secret offshore accounts.
"I've been there. I've done the job. I know what is the day-to-day business," he said, explaining why he thought it important to identify himself as the source.
The banker, who worked as Julius Baer's chief operating officer in the Cayman Islands, said he and his family faced pressure akin to "a fire-breathing dragon with several heads" after he decided to blow the whistle.
He said he was put in prison in Switzerland for 30 days for violating Swiss banking rules, and that he was offered money and the withdrawal of charges against him in order to buy his silence.
The data included the offshore accounts of about 40 politicians, he said, and covered accounts at three banks, including his former employer.
The banker also said that he and his wife had written a letter to German Finance Minister Peer Steinbruck, offering to provide the data for free, but received no response.
Mr Elmer - who runs his own whistle-blowing website - said the data he was providing had been passed to him by various sources that he would keep anonymous.
"I am taking the responsibility for this," he said.
He is already facing trial in Switzerland for a previous data leak, and has admitted breaking some laws.
However, Mr Elmer claimed he did not breach Swiss banking secrecy rules, as he said all of the information related to the Cayman Islands and therefore lay outside Swiss jurisdiction.