A senior aide to South Korean President Lee Myung-Bak offered to resign Friday amid allegations of involvement in a bribery scandal which has rocked the ruling party in an election year.

The offer by senior political affairs secretary Kim Hyo-Jae came one day after parliament Speaker Park Hee-Tae stepped down over the affair, which is being investigated by prosecutors.

Park or his aides are alleged to have offered cash envelopes to lawmakers of the conservative Grand National Party before a vote -- which Park won -- to select a new party chief in 2008.

Park, who quit the party post before becoming Speaker in 2010, took responsibility for the affair but did not admit personal wrongdoing.

Kim was an aide to Park at the time.

"I am sorry for causing concern to the public... I will take all of my political responsibility," a presidential spokesman quoted Kim as saying, according to Yonhap news agency.

Lee is widely expected to accept the resignation after returning Saturday from a Middle Eastern trip, Yonhap said.

Since the affair was publicised in early January by a whistle-blowing legislator, Kim has denied any involvement in the suspected bribery.

The lawmaker said he received an envelope stuffed with three million won ($2,685) from an aide to the Speaker, which he returned.

The disclosure dealt a blow to the conservatives, already suffering from waning support.

The party now holds 166 of the 299 parliamentary seats along with the presidency. But it anticipates a struggle in the April general election and the presidential poll in December.

Recent surveys show the main opposition Democratic United Party is more popular than the ruling party amid growing discontent over social and economic inequality and an economic slowdown.

The embattled conservatives, in a bid to shed their image as the party for the rich, have shifted policies leftward to focus on welfare for the poor.

The Grand National Party also changed its name -- to the Saenuri party -- a common rebranding tactic in Korean politics. The party said Thursday its name in English would be the New Frontier Party.