Prosecutors have asked former Democratic Party of Japan Secretary General Ichiro Ozawa to submit to voluntary questioning following a judicial panel's decision that the move not to indict him over alleged false financial reporting involving his fund management body was unreasonable, sources said Friday.

If the DPJ's former No. 2 complies with the request, it would be the fourth time for him to be questioned by prosecutors over a scandal that has continued to rock the DPJ-led coalition government.

The Tokyo No. 1 Committee for the Inquest of Prosecution, which was reviewing the fund management body's alleged falsification of its 2007 report, said in its July 8 decision that past questioning of Ozawa was of insufficient depth.

There were reservations among prosecutors about questioning the powerful lawmaker again, but the majority opinion was that he should be quizzed in line with the independent panel's decision, the sources said.

The prosecutors have already requested that three former Ozawa aides submit to questioning on a voluntary basis. One of them, House of Representatives member Tomohiro Ishikawa, has expressed readiness to comply, while another has refused, according to the sources.

The No. 1 panel noted in its decision that the alleged donation of shady funds from Mizutani Construction Co. in Kuwana, Mie Prefecture, had "serious connections" with the presumed motives behind the false financial reporting.