Mr Chung (centre) apologised for his gaffe

Chung Dong-young had been criticised for saying that older voters could stay at home on election day, in an attempt to encourage younger voters.

The party has since seen its approval rating dip.

It had been boosted by sympathy for its backer, President Roh Moo-hyun, who was impeached last month.

Mr Chung gave no reason for his resignation at a hastily convened news conference on Monday, according to Reuters news agency.

But his remark about older voters had attracted widespread opprobrium in a society dominated by the Confucian culture, which honours seniority.

He had hoped to attract South Korea's 20- and 30-somethings to vote, who have traditionally shown less interest in politics.

Mr Chung also said he would not seek a seat in the National Assembly in Thursday's election.

Before his verbal gaffe, the minority party with only 47 seats in the outgoing National Assembly had been leading the conservative GNP by a margin of 2-1 in opinion polls.

But the GNP has since recovered from a backlash against the party following Mr Roh's impeachment.

"The party will do its utmost to win the elections up until the last minute on the day after tomorrow," Mr Chung said.

The elections are widely seen as a referendum on Mr Roh, who was censured by parliament on 12 March for violating election rules by saying he would like his supporters in the Uri party to do well.

The Constitutional Court has up to six months to decide whether to back the national assembly's action.