Roh Moo-hyun took over as president last December

His decision was widely expected, after many of his parliamentary allies defected to launch an independent reformist group earlier this month.

Mr Roh did not say whether he would eventually join the new group.

But his spokesman Yoon Tai-yong said he would remain unaffiliated to any party until the end of the year.

Despite being a minority party in the South Korean National Assembly, the MDP has until now been considered the ruling party because of the president's membership.

According to Mr Yoon, that will no longer be the case.

Mr Roh announced his decision to quit through a statement read by his spokesman on Monday.

"I made up my mind to stop my MDP membership from causing any further political debate," he said, adding that he wanted to concentrate on state affairs rather than party politics.

Mr Roh won last December's presidential elections as the MDP candidate, as did his predecessor Kim Dae-jung, who founded the party.

But in recent months, the MDP has been hit by corruption scandals and bitter factional divides, and Mr Roh's loyalists have spent much of their time struggling to reform it in time for the April elections.

They finally gave up nine days ago, when 37 lawmakers broke away from the party to form a new group.

Many of Mr Roh's allies urged him to follow suit, after many MDP members joined opposition parties to vote against the president's nominee to run the audit bureau last week, highlighting the rift in the party.

Following the recent defections, the MDP has been left with only 64 seats out of 272 in the National Assembly, which is controlled by the opposition Grand National Party.

As well as the 37 original defectors, the new party has already attracted five other reform-minded lawmakers, and has become the third largest party in the assembly.