Japanese Prime Minister Junichiro Koizumi's troubled ruling party has suffered a new blow with the resignation of the speaker of parliament's upper house.

Yutaka Inoue resigned over allegations that his secretary received a $492,000 (64 million yen) bribe from the president of a construction company in connection with a public works project.

Mr Inoue, 74, has denied the allegations, but stepped down after opposition parties threatened to boycott parliament.

It is the latest in a series of money scandals that have tainted Japanese politics.

Mr Inoue's resignation has particularly hit the ruling Liberal Democratic Party, to which he belonged until 2000, when he assumed the post of president of the House of Councillors.

Our correspondent in Tokyo says Mr Inoue lost the support of party heavyweights.

Financial scandals

Earlier this month, Koichi Kato, a former influential MP for the LDP, resigned from parliament over alleged misuse of political donations.

And in March, an outspoken opposition MP, Kiyomi Tsujimoto resigned after she was accused of channelling government money that was meant to pay the salary of a political secretary, to cover office expenses.

Earlier this year, a prominent LDP politician, Muneo Suzuki, was forced to step down over revelations that he wielded enormous influence behind the scenes over foreign ministry bureaucrats.

The rash of political scandals have knocked Mr Koizumi's popularity ratings. He swept to power a year ago promising to clean up politics and revive the economy.

Opinion polls show put support for him at 40%, down from 90% highs when he first took office.

His popularity has also taken a beating since he sacked his popular Foreign Secretary Makiko Tanaka after she became entangled in a feud with diplomats.