A former head of South Korea's biggest opposition party kept a comfortable lead in a presidential opinion poll over former UN chief, who returned back to his home country, a local survey showed on Friday.

According to a Gallup Korea poll conducted from Tuesday to Thursday, 31 percent respondents supported Moon Jae-in, former chief of the main opposition Minjoo Party.

His approval ratings far exceeded the 20-percent score garnered by Ban Ki-moon, who arrived in South Korea on Thursday from New York after ending his second, five-year term in the top UN post.

Local media outlets speculated that Ban indicated his run for the presidency though he has never declared it officially.

Ban has long been seen as the best hope in the conservative bloc as support for conservative politicians sank after the impeachment of President Park Geun-hye. If the constitutional court upholds the impeachment, a presidential election must be held in 60 days.

Lee Jae-myung, mayor of Seongnam city to the southeast of Seoul who is affiliated with the Minjoo Party, was at the third place in the recent presidential survey with 12 percent.

Ahn Cheol-soo of the minor opposition People's Party ranked fourth with approval scores of 7 percent.

In a three-way scenario of presidential election, the former Minjoo Party chief would be elected as the next leader with 44 percent in support, far exceeding the former UN chief's 30 percent and Ahn's 14 percent.

In the simulated showdown between Moon and Ban, the former won 53 percent, greatly beating the latter's 37 percent.

Meanwhile, support rate for the Minjoo Party was 41 percent. It surpassed the ruling Saenuri Party's 12 percent, the People's Party's 10 percent and the Righteous Party's 7 percent.