SEOUL, Jan. 5 (Yonhap) -- The ruling Grand National Party (GNP) said Thursday it will request prosecutors to look into allegations that one of its former party leaders bribed fellow party members to get the chief post in the past leadership election, dealing yet another blow to the struggling party ahead of major polls.

The GNP's move comes after Rep. Ko Sung-doug Wednesday disclosed that an aide to one of the then chairperson candidates had handed in an envelope stuffed with 3 million won ($2,600) before the party's leadership vote. The lawyer-turned-lawmaker said he immediately returned the money and the candidate later became the party's new leader.

Though Ko did not name a specific person, two former chiefs considered close to President Lee Myung-bak were mentioned as a potential briber in local media. 

There have been speculations that some candidates tried to bribe fellow party members to get the chief post in the party contests, but it is the first time that a GNP lawmaker raises specific allegations in regard to the vote.

On Thursday, the GNP's interim leader Park Geun-hye called for a prompt investigation to leave no suspicions to people in regard to the party's leadership vote.

The party's leading presidential contender took helm of the party's emergency council last month with pledges to sweeping reforms, after ex-chairman Hong Joon-pyo stepped down under pressure from reformist lawmakers.

"We've decided to request prosecutors to look into allegations revealed by Ko within the day," the party's spokesman Hwang Young-chul said at a briefing. "Our decision aims to revamp the distorted political culture."

Ko said he will fully cooperate with the investigation if summoned.

Opposition parties ratcheted up attacks against the ruling party, which has been stained by a series of scandals involving its lawmakers.

"While allegations of corruption are unfolding regarding the president's aides day after day, bribery in the process of the GNP convention was exposed," the Democratic Unity Party spokesman Oh Jong-sik said in a statement.

Adding to the woes was a widening prosecution investigation that an aide to one of the GNP's lawmakers was arrested on charges of masterminding a hacking attack on the Web site of the national election watchdog on the day of the October by-elections. Rep. Choi Ku-sik denied his involvement in the scheme, but left the party amid growing pressure from his own party weary of a negative image from the incident ahead of the April parliamentary elections. 

On Thursday, the party's emergency council launched a civilian committee to verify whether the ongoing prosecution investigation is fair and transparent, as part of efforts to dispel lingering suspicions that higher ranks in the GNP may have been involved in the hacking.