Toshikatsu Matsuoka was embroiled in a financial scandal

The 62-year-old was found hanged in his Tokyo apartment hours before he was to face questions in parliament over his links to a political funding scandal.

Opposition MPs had been calling for his resignation over unexplained expenses.

PM Shinzo Abe expressed regret over the news saying he was conscious of his "responsibility as prime minister, and as the one who appointed him".

"The effects on the cabinet will be great," Mr Abe added as he went to attend a wake for the deceased minister late on Monday.

Mr Matsuoka was found unconscious in his flat around lunchtime on Monday. Efforts to resuscitate him at hospital failed.

Police said he had hanged himself, but refused to comment on reports in the Japanese press that he had left a number of suicide notes in his apartment.

It is the first time a Japanese cabinet minister has killed himself since World War II, when the army minister committed suicide on news of Japan's surrender.

Funding questions

Mr Matsuoka was embroiled in two political scandals, though he denied any wrong-doing.

He had allegedly claimed more than 28m yen ($236,600; £118,300) in utility fees at his parliamentary office, where utility costs are free.

It was also alleged that he had received electoral campaign donations from a businessman linked to a bid-rigging scandal.

Mr Abe had vigorously defended his minister, often against the advice of some within his own party, saying he had fulfilled his legal responsibilities.

The BBC's Chris Hogg in Tokyo says Mr Matsuoka's death is another blow for Mr Abe's government.

Even before news of Mr Matsuoka's death emerged, opinion polls published on Monday suggested support for Mr Abe's administration was already at the lowest point since he took office eight months ago.