SEOUL (BLOOMBERG) - For about as long as North Korea has existed, Kim Pyong Il has been considered a possible successor to the throne. And now, with his nephew Kim Jong Un's health status unclear, his name is being bandied about again.

Mr Kim Pyong Il, 65, is the last known surviving son of North Korea's founder, Kim Il Sung. After losing out in the 1970s to his half-brother, Kim Jong Il - who ended up running the country from 1994 to 2011 - Mr Kim Pyong Il spent about four decades overseas in diplomatic posts including in Hungary, Bulgaria, Finland, Poland and the Czech Republic before returning to Pyongyang last year.

Although Mr Kim Pyong Il has been effectively sidelined - he was largely purged from state media and never developed enough power back home to mount a serious challenge for leadership - some North Korea watchers say he could end up taking over from the 36-year-old Kim Jong Un, who hasn't named a successor. This is mainly because he has Kim blood, and he's a man.

The conservative male leaders in Pyongyang would resist giving power to Ms Kim Yo Jong - Mr Kim Jong Un's younger sister who has been by his side helping to make policy the past few years - according to Mr Thae Yong Ho, who was North Korea's deputy ambassador to the UK before he defected to South Korea in 2016. That's due to her gender and relatively young age of her early 30s.

"The problem is that a Kim Yo Jong-led North Korea is unlikely to be sustainable," Mr Thae said, warning that collective leadership with her as the figurehead could lead to chaos. "To avoid this, some in the leadership would try to bring back Kim Pyong Il, who's now under house arrest, to the centre of the power."

Others don't think Mr Kim Pyong Il has a chance. South Korean ruling party lawmaker Kim Byeong-ki, a member of Parliament's intelligence committee, said on Sunday on social media that there was no indication he could possibly succeed Mr Kim Jong Un if the leader were incapacitated: "I laugh off these theories."

North Korea has often exiled those who fall out of favour, sending them abroad in attempts to erase their influence, but also providing a financial lifeline that keeps them dependent on Pyongyang's rulers.

If Mr Kim Pyong Il took power, it could put a great number of the current top leadership in jeopardy after they spent decades working to suppress his influence. When Mr Kim Jong Un took power after his father's death in 2011, he soon eliminated potential rivals: He executed his uncle and one-time deputy, Mr Jang Song Thaek, and was suspected to have ordered the assassination of his exiled older half-brother, Mr Kim Jong Nam, in Malaysia.

The fact that Mr Kim Pyong Il survived the purges in the ruling family may indicate that Mr Kim Jong Un never saw him as a credible rival, keeping him in the foreign service and at arm's length for years. In 2015, he was named North Korea's ambassador to the Czech Republic and was given extra protection in 2017 when Mr Kim Jong Nam was murdered.

Mr Kim Pyong Il kept a low profile while he was in Europe, though he still made an impression. Mr Lubomir Zaoralek, who was the Czech Republic's foreign affairs minister from 2014 through 2017, said "his style and manner were as if he had come from South Korea".

"You could see that he was established in Europe and that he has lived his life here," Mr Zaoralek said. "He was always careful in what he had to say, but it always made perfect sense. And it seemed that he lived a much freer life here than other North Koreans."

Mr Kim Pyong Il returned to Pyongyang last November, so that Mr Kim Jong Un could keep a closer watch on him, the JoongAng Ilbo newspaper reported, citing intelligence sources.

He has been the subject of speculation for decades in South Korea in unverified reports about family intrigue, often including house arrests and attempted assassinations. Before his years abroad, he served in the army, commanding an elite body guard unit, and also was appointed to posts in the ruling party, according to the South Korean Unification Ministry.

His mother Kim Song Ae - the second wife of the state founder - was influential in the 1970s and pushed for Mr Kim Pyong Il to take power. But she soon fell out of favour after Mr Kim Jong Il was named successor.

Mr Kim Pyong Il is largely seen as a contender this time by those who discount Ms Kim Yo Jong due to her age and gender, according to Ms Rachel Minyoung Lee, a former North Korea analyst with the United States government.

"It is highly unlikely that he has the connections or the support base he needs to be the next North Korean leader," she said. "Kim Yo Jong has a special status in the regime, and I think in this case, her connection to the Kim family trumps her gender."