The Yonhap news agency said Ban made the announcement during an unscheduled press conference on Wednesday at South Korea's parliament building after meeting the leaders of several conservative parties.

"I will withdraw from politics," he told reporters. "I'm sorry for disappointing many people."

Ban said he was disappointed at the "selfish ways" of some politicians in his home country, adding that it was "meaningless" to join them.

He also hinted at his disillusionment with an attempted smear campaign since his name was linked with running for the presidency, after what he said was "fake news" about his personal life sometimes made him appear out of touch, and corruption allegations were leveled at some of his relatives.

"My pure patriotism and aspirations have fallen victim to slander that was close to personality slaughter," he said.

Surprise u-turn

Ban, who stepped down at the end of last year after a decade as UN chief, had returned to South Korea on January 12 and was widely expected to run in elections due this year.

As one of the most recognized South Koreans, he initially had strong public opinion poll ratings as a possible candidate to succeed impeached President Park Geun-hye,  who is engulfed in a corruption and influence peddling scandal.

But local media said Ban's approval ratings have been falling in recent weeks as he tested the waters with a series of public appearances before announcing his candidacy.

Polls suggested his support had dropped from 20.3 percent when he returned from New York to 13.1 percent before Wednesday's announcement, leaving him in second place to the main Democratic United Party candidate Moon Jae-In.

Ban was widely expected to join Park's ruling Saenuri party, or an emerging conservative breakaway.

A career diplomat, the former UN chief previously never joined any South Korean political party, although he served as foreign minister under the late liberal president Roh Moo-Hyun from 2004 to 2006.

Elections could be called soon

Park's close aide Choi Soon-sil is on trial for alleged extortion of South Korea's large business conglomerates.

Park was temporarily removed from office by parliament on December 9 over her alleged links to the scandal, and the Constitutional Court will rule as early as this month whether to make it permanent. Elections must be held within 60 days of that ruling.

mm/kms (AFP, AP, Reuters)