Thae Yong-Ho, who fled his post as North Korea's deputy ambassador to Britain in August, said: "I'm sure and I can say that Kim Jong-Un's days are numbered."
In his first press conference for foreign correspondents, held under tight security, Thae said he was sure that more of his fellow countrymen would follow suit since North Korea was "on a downward path".
The elite were "turning their backs" on leader Kim Jong-Un, he said, adding: "The traditional structures of North Korean systems are crumbling."
Nuclear-armed North Korea has been ruled by the Kim dynasty since its foundation in 1948.
It is subject to United Nations Security Council sanctions over its nuclear and missile programmes and is accused by the West of widespread human rights abuses.
Thae is among the highest-ranking defectors from the North for years. He said he had begun to waver as his diplomatic role granted him access to outside information.
Former North Korean deputy ambassador to the UK, Thae Yong Ho speaks with media at the Foreign Correspondents Club in Seoul, South Korea, Jan. 25 (AAP) Source: AAP
His disillusionment turned to despair after Kim, who inherited power from his late father Kim Jong-Il five years ago, began ruthlessly purging officials, he added.
Kim had his own uncle and one-time political mentor Jang Song-Thaek executed in late 2013 on an array of charges, including treason and corruption.
Kim was installed as chairman of the State Affairs Commission -- a new supreme governing body -- in June, a month after his Workers' Party held its first congress in 36 years in what was widely seen as his coronation.
North Korean diplomats are generally compelled to leave one of their children behind in Pyongyang when they are dispatched abroad, but Thae was able to take both his sons, now aged 19 and 26, to London -- easing his preparations to defect.
"The Kim Jong-Un regime abuses love between parents and children to control North Korean diplomats," he said.
After Thae's defection the North's state media denounced him as "human scum" and accused him of embezzling state funds, raping a minor and spying for South Korea in exchange for money.
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Pyongyang carried out two nuclear tests and a series of missile launches in 2016 and Thae said Kim was planning to "complete" its atomic development by the end of this year to take advantage of leadership transitions in South Korea and the United States.
"The only way to resolve the issue of North's nuclear threats is the elimination of Kim Jong-Un's regime," he said.
He called for continued international sanctions on Pyongyang and publicity campaigns to spread external information in the North and encourage its citizens into "popular uprisings".
Some tentative economic reforms have been put into effect in the North, but "those further down the food chain are finding life much tougher", Thae said.
Once "unthinkable" acts of low-level dissent or criticism were becoming more frequent.
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