Prosecutors investigating the funding scandal of Democratic Party of Japan heavyweight Ichiro Ozawa have apparently ditched plans to use as evidence a record of the interrogation of his former aide because a disgraced Osaka prosecutor charged with evidence tampering was involved in the questioning, sources said.

The prosecutors are believed to have made the rare decision out of concern that the credibility of the statement by Takanori Okubo, one of Ozawa's former aides, could be questioned, the sources said.

The document was based on interrogations of Okubo, 49, carried out by then prosecutor Tsunehiko Maeda before the ex-aide was indicated last February in connection with Ozawa's political funds scandal.

Maeda, who was with the Osaka District Public Prosecutor's Office at that time, was charged with evidence tampering in a separate case that arose last September.

The prosecutors conveyed the retraction of the document during Thursday's session on pretrial procedures held at the Tokyo District Court for the upcoming trial of three former secretaries to Ozawa, including Okubo, who have been charged with violating the Political Funds Control Law, the sources said.

Maeda allegedly tampered with evidence when he grilled a welfare ministry bureaucrat charged with issuing a fabricated ministry document. The bureaucrat was later acquitted.

Ex-Ozawa aides Okubo, House of Representatives lawmaker Tomohiro Ishikawa, 37, and former private secretary Mitsutomo Ikeda, 33, were charged last February with failing to enter ¥400 million allegedly provided by Ozawa for the purchase of land in Tokyo in the 2004 report of the DPJ kingpin's Rikuzankai funds management body.