Mr Shin asked for forgiveness on behalf of his father

Shin Ki-nam had earlier said his father worked as a teacher during that period.

But at an emotional press conference, he accepted reports his father had worked for Japan's military police.

Mr Shin's resignation coincides with a call by President Roh Moo-hyun for an inquiry into those who collaborated with the Japanese colonial power.

President Roh brought the issue into the public eye during a speech last weekend marking the 59th anniversary of liberation from Japanese rule.

He spoke of the need for debate and soul-searching about the colonial period, which has never been intensely investigated in South Korea.

The opposition Grand National Party claimed the campaign was targeted at its leader Park Keun-hye, the eldest daughter of former President Park Chung-hee, who served as a Japanese army officer.

But Mr Roh said those whose ancestors had collaborated with the colonial power - or with the military government that succeeded it in the 1960s - must not face punishment.

'Forgiveness'

Shin Ki-nam has been an enthusiastic supporter of the plan to investigate collaborators, and backed the setting-up of committees to name and shame those who worked for the Japanese more than six decades ago.

But now he has been forced to face up to his past. Two veterans of the independent struggle have come forward to say they were tortured by Mr Shin's father.

Mr Shin told reporters he was stepping down so as not to "injure the [party's] determination to clear up the history of Japanese collaboration".

"I still find it shocking and difficult to believe details of recent reports about my father," he said.

"In place of my father, I deeply apologise and ask forgiveness," he said, vowing to help the country continue to answer questions about its past.

The BBC correspondent in Seoul, Charles Scanlon, says South Korea has never come to terms with the widespread collaboration that took place during the Japanese occupation.

Party problems

Mr Shin's resignation throws the leadership of the Uri Party into confusion.

He has been replaced - at least temporarily - by Lee Bu-young, a veteran member of parliament who lost his seat in the 15 April parliamentary election.

Mr Lee, who switched to the Uri Party from the Grand National Party late last year, will hold the job until the party calls a convention to elect a leader.