It stymies earlier efforts by the party whips to resume the operation of the parliament at an impasse for nearly three months after wrangling over a push to place key reform bills on a fast-track.
LKP floor leader Na Kyung-won said her party, after holding a general meeting, concluded that the agreement lacked specifics and therefore found it difficult to approve.
"(The initial deal) was agreed to be proceeded with only on the condition that we had the party's approval. And there was a call from members of our party that the agreement needs to be more clear," she told reporters.
Monday's move also puts in limbo the parliamentary discussion for a 6.7 trillion-won ($5.6 billion) extra budget bill and other legislation on people's livelihoods that have been pending for months amid deepened partisan tensions.
The National Assembly held its first plenary session Monday since April 5, but in the LKP's absence.
During the session, Prime Minister Lee Nak-yon made a budget speech, calling for the quick passage of the bill to enable the government to carry out additional fiscal spending no later than in July.
"I believe ruling and opposition parties do not have different mindsets in sharing the need to prevent an economic contraction, ease the people's plights and boost safety measures," Lee said.
Earlier in the day, the ruling Democratic Party (DP), the main opposition Liberty Korea Party (LKP) and the minor opposition Bareunmirae Party (BP) reached a last-minute deal to set detailed schedule for an extra session in June and put the idle parliament back on track.
They decided to kick off the review of the supplementary budget bill and pass it during the June session. The parties also agreed to put priority on reviewing the budget allocated for disaster controls.
Partisan tensions started after the DP and three minor parties placed key political and judicial reform bills on a fast-track in late April, despite strong objections from the LKP.
The contentious bills included an election reform bill, a proposal to set up a special unit to probe high-ranking public officials and a measure to adjust the investigative power between the prosecution and the police.
"As for the bills designated as fast-track ones, the parties will comprehensively discuss each party's proposals and handle them in accordance with the spirit of agreement," the three parties said in a joint statement.
Anticipation for a breakthrough had come amid mounting criticism that the National Assembly is embroiled in political strife while neglecting its legislative role.
The DP and the three minor parties -- the BP, the liberal Party for Democracy and Peace and the leftist Justice Party -- made a joint request to hold the June session last week as the impasse was drawn out.
The one-month June session kicked off Thursday, but without the participation of the LKP, the assembly could not fully resume operation.
The LKP had recently demanded the parliament hold hearings on economic issues, claiming it wants to zero in on the failure of the government's economic policy before it starts to review the extra budget bill. The DP rejected the LKP's demand as "rude."
Instead of the format of hearings, the rival parties agreed to accept the holding of roundtable discussions on the economy, to be hosted by National Assembly Speaker Moon Hee-sang. Details will be set by the parties' negotiations.
The floor leaders of the three parties apologized to the public over the protracted deadlock.
"It is very regrettable that the parliament has repeated the crippled operation for a long time. As we've decided to normalize its operation, we will do our best for legislative efforts," DP floor leader Lee In-young told reporters.
For the budget bill review, heated debate is expected as the conservative LKP claims the extra budget is designed to create jobs to win over voters ahead of next April's parliamentary elections.
The government submitted an extra bill to the assembly on April 25. The bill is aimed at coping with fine dust and boosting the slowing economy amid growing economic downside risks at home and abroad, including a trade war between the United States and China. (Yonhap)