TOKYO - A scandal-tainted minister in Prime Minister Shinzo Abe's cabinet committed suicide on Monday, compounding problems for the Japanese leader whose support has slumped ahead of a July election.

It was the first suicide by a Japanese cabinet minister since the days after Japan's defeat in World War Two, according to officials at the national library.

"This will have serious political fallout, but at this point it's hard to tell how much," a government official told Reuters.

Agriculture Minister Toshikatsu Matsuoka's suicide came as Abe's public support rate fell to its lowest level since he took office last September, due largely to voter anger over mismanagement of pension premiums that could shortchange retirees.

The dent in Abe's popularity had already increased chances that his ruling camp would lose its majority in the election for parliament's upper house, his first big test at the polls.

Matsuoka, 62, under fire for a series of political funding scandals, died in hospital after he was found unconscious in his room at a Tokyo residential complex for lawmakers.

Police said he hanged himself but declined to comment on Japanese media reports that he left five or six suicide notes.

Kyodo said one note was addressed to Abe and that another one with no addressee said: "I am sorry for causing trouble".

A visibly-shaken Abe told reporters: "I am overwhelmed with shame. I deeply hope his soul will rest in peace."

Matsuoka, who had repeatedly denied any wrongdoing, had been scheduled to be grilled again in parliament later on Monday.

Critics had charged that Abe was protecting Matsuoka, and the prime minister's image would likely suffer in the short term, political analysts said. But they added that the long-term fallout could depend on how Abe handled the matter.

PENSION WOES

"It's hard to say what will happen. It depends on Abe's response," said Jun Iio, a political science professor at the National Graduate Institute for Political Studies.

"This could make it hard for the opposition to follow up on the scandals, but it could also give the impression that there was something so bad that he had to commit suicide."

Opposition leaders expressed shock at Matsuoka's suicide.

"It's regrettable that Prime Minister Abe didn't make (Matsuoka) explain himself to the public," Mizuho Fukushima, leader of the tiny Social Democratic Party told reporters. "The prime minister has heavy responsibility over this."

Japan has one of the highest suicide rates among industrialised nations, which experts attribute partly to an absence of religious prohibition against taking one's own life and the tradition of committing suicide to atone for failure or to save loved ones from embarrassment.

Abe's administration was already under fire after the Social Insurance Agency, which manages the pension system, acknowledged that it had failed to keep proper track of 50 million premium payments.

Abe has said his government would try to sort out the problem and ensure full payments were made, but voters appeared unconvinced.

Only 32 percent of the voters who responded to a weekend survey by the Mainichi newspaper backed Abe, down 11 points from April, while a poll by the Nikkei business daily put the prime minister's support rate at 41 percent, down 12 points.

The Mainichi survey showed that 42 percent of the voters want the main opposition Democratic Party to win the July election, compared with 33 percent who want Abe's ruling Liberal Democratic Party (LDP) to win.

The LDP and its junior coalition partner, New Komeito, need to win a total of 64 seats out of 121 up for grabs in the July election to keep their majority in the 242-seat upper house.

Loss of a majority in the upper house would not require Abe to resign, since the lower chamber picks the premier. But it could increase calls within his party to step down and would also mean that the opposition could block key legislation and delay economic reforms.

Legislative deadlock could force the prime minister to call a snap election for the lower house, analysts said, although no lower house election is required until 2009.