Three ex-prime ministers under the previous Moon Jae-in administration have expressed concerns over the deepening divide within the main opposition party and throughout the liberal bloc as of Tuesday.

Former Prime Ministers Lee Nak-yon, Chung Se-kyun and Kim Bu-gyeom pointed to the Democratic Party of Korea's current leader Lee Jae-myung's monopolization of power within the party as a major hurdle to party unification ahead of next year's general elections, in two separate meetings held this week.

Lee Nak-yon, who is also the Democratic Party's ex-leader, met with Chung behind closed doors on Tuesday to discuss "the existing issues surrounding the party."

"We shared concerns surrounding the Democratic Party and national affairs," the party's ex-leader said in a statement released after the meeting.

"If the situation allows, we plan to hold a meeting of the three prime ministers under the Moon administration (to discuss the imminent issues)," he added.

Tuesday's meeting closely follows similar talks between Chung and Kim held Sunday.

The two ex-prime ministers voiced concerns about Lee Jae-myung's apparent discrimination against his own party members in the candidate registration process for the upcoming election.

"(Chung and Kim) expressed worries that such issues tied to the election could only fuel further divide within the Democratic Party," an official close to the matter said, requesting anonymity.

Last week, the Democratic Party's election candidate monitoring committee rejected three of its own lawmakers' requests to run in the general election. The three candidates were known to have either criticized Lee Jae-myung in the past or refused to work with him as close aides. A group of reform-minded lawmakers within the party openly criticized the committee, saying that they "doubt the fairness of the decision."

Meanwhile, Chung plans to meet Lee Jae-myung on Thursday, in which a Democratic Party spokesperson said the former prime minister plans to share his concerns and the outcomes of Tuesday's meeting.

Meanwhile, Lee Nak-yon has been calling for intraparty reform centered on Lee Jae-myung's resignation from his current leadership post and establishment of a "unified" emergency leadership committee. The former prime minister warned that he could potentially leave his current party and launch a new political party of his own if Lee Jae-myung fails to respond to his request.