The speaker of South Korea's parliament resigned on Thursday over bribery allegations involving lawmakers from the ruling party in the latest graft scandal to hit the struggling conservatives before a general election in April.

Park Hee-tae's resignation deals another blow to President Lee Myung-bak and his governing party, once seen as sure-fire winners in a rare combined parliamentary and presidential poll, but now lagging the left-leaning opposition.

Prosecutors are investigating a lawmaker's claim that Park tried to bribe fellow conservatives to become the head of the then Grand National Party three years ago.

The whistle-blowing lawmaker said a man allegedly representing Park had carried a shopping bag chock-full of yellow manila envelopes containing cash to try to bribe lawmakers in bid to win the chairmanship of the party.

I apologise to the people, Park said in a statement released by his office. I will take it all on my shoulders. If there are people involved (in the scandal), it's all my responsibility.

The prosecutor-turned-lawmaker dropped his party affiliation to become the parliamentary speaker in 2010. The speaker's two-year term was scheduled to end in May.

It is the first time a legislative chief has stepped down over an investigation by prosecutors into a bribery scandal, media reports said.

His resignation in the absence of formal legal charges, and less than two weeks after another of Lee's high-profile allies quit in another graft case, is seen as an attempt to shield the ruling conservatives from further political damage.

Choi See-joong stepped down as chairman of the Korea Communications Commission, but admitted to no wrongdoing personally or by associates. He has been called Lee's political mentor, playing a key role in his 2007 election win that ended two consecutive left-of-centre presidencies.

Despite years of efforts to clean up the country's image as a place where bribes and backroom dealing are prerequisites to getting business done, graft scandals remain a regular feature of bitter politicking.

(Reporting by Jeremy Laurence; Editing by Jack Kim and Ed Lane)