Former UN secretary-general Ban Ki-moon announces on Wednesday he will not seek South Korea's presidency. [Photo/Son Hyung-ju via Reuters]

Former United Nations secretary-general Ban Ki-moon abruptly ended his attempt to seek South Korea's presidency on Wednesday, dropping a lifetime of diplomacy to denounce his country's political establishment.

Ban, 72, returned home last month after a decade in New York and was widely expected to run in elections due this year, but his anticipated candidacy ran into a series of stumbles and he struggled for backing.

"I will give up my pure intention to bring about a change in politics under my leadership and to unify the country," he said at a news conference. "I'm sorry for disappointing many people."

His support in public opinion polls had rapidly declined from 20.3 percent when he returned to 13.1 percent before his announcement.

Although he never officially declared he was running, the former UN chief embarked on a series of public appearances.

But he struggled to secure party backing in South Korea's highly partisan political system, and corruption allegations were made against some of his relatives.

Pictures of him trying to put two bank notes into a ticket machine at the same time made him appear out of touch.

"My pure patriotism and aspirations have fallen victim to slander that was close to character assassination," he said Wednesday, before bowing briefly, shuffling his papers and leaving the stage to a barrage of camera flashes.