TOKYO - Japan's former opposition leader faced a fresh blow Friday when a businessman admitted in court he had given illegal donations to the politician's office in return for public works contracts.

Tokyo prosecutors have charged that the office of lawmaker Ichiro Ozawa -- a veteran politician who last month resigned as the leader of the opposition Democratic Party of Japan (DPJ) -- had accepted the illegal donations.

The scandal has rocked Japanese politics in an election year when the DPJ hopes to topple Prime Minister Taro Aso's Liberal Democratic Party (LDP), which has ruled Japan almost without break for more than half a century.

The illegal corporate funds, which were allegedly channelled through front groups, were intended to sway Ozawa to award lucrative public works contracts in his northern Japan constituency, prosecutors have claimed.

The man on trial, former Nishimatsu Construction president Mikio Kunisawa, pleaded guilty in Tokyo District Court to ordering his firm to make the donations and to illegally bringing undocumented funds from overseas.

Ozawa is not indicted and has insisted that he has not broken any law.

But prosecutors have used Kunisawa's case to claim that Ozawa was the political operator in charge of awarding the public works contracts and gave directives euphemistically known as "voices from heaven."

Ozawa's top aide, Takanori Okubo, has been indicted for allegedly asking for and accepting donations from Nishimatsu through front groups.

Ozawa refused to comment on the trial when quizzed by reporters Friday.

Prime Minister Aso, who faces sagging public support and has to call elections by October, told journalists: "The public hopes the relationship between money and politics will become clear in court."

Ozawa led over Aso in opinion polls for months before Okubo's March arrest, but support for both him and the party plunged subsequently, forcing Ozawa to resign the party leadership, although he kept his parliamentary seat.

The DPJ is once more leading in polls under a new party chief.

A similar money scandal involving Nishimatsu also touched Industry Minister Toshihiro Nikai and other senior LDP lawmakers, but prosecutors have not indicted any of them.

A citizens' legal review panel last week called on prosecutors to revisit the issue to build criminal cases.