Ruling party presidential candidate Lee Jae-myung and his main opposition rival Yoon Suk-yeol are in an increasingly tight race, with one opinion poll putting Lee ahead of Yoon and another survey showing contrary results.

In a Korea Society Opinion Institute (KSOI) survey of 1,008 adults, Lee led with 40.3 percent against Yoon's 37.4 percent, though the gap was within the margin of error of plus or minus 3.1 percentage points at the 95 percent confidence level.

But a Realmeter poll of 3,043 people put Yoon ahead with 44.4 percent against Lee's 38 percent. Their 6.4 percentage point gap is outside the margin of error of plus or minus 1.8 percentage points, with a 95 percent confidence level.

In the Realmeter poll, both Lee and Yoon saw their support ratings drop 1.7 and 0.8 percentage point, respectively, in the wake of allegations that Lee's son gambled illegally and Yoon's wife falsified credentials on her resume.

Ahn Cheol-soo, the presidential candidate of the minor opposition People's Party, came in third with 3.9 percent, followed by Sim Sang-jeung of the minor progressive Justice Party at 3.2 percent in the Realmeter survey.

Ahn and Sim garnered 4.6 percent and 4.2 percent, respectively, in the KSOI survey.

Meanwhile, support for the PPP stood at 39.2 percent, while the DP had 33.1 percent, the Realmeter survey showed.

President Moon Jae-in's job performance received 40.2 percent approval and 56.8 percent disapproval. (Yonhap)