New documents unearthed in Tokyo reveal that North Korean leader Kim Jong-un's maternal grandfather worked for the Japanese military, sparking claims that the reclusive state's leader is the descendant of a traitor.

Japanese human rights activist and researcher Ken Kato says the relative, Ko Gyon-tek, made uniforms for the Japanese army at the Hirota sewing factory in the Japanese city of Osaka, making him - under North Korean law - a collaborator.

In the Orwellian world of North Korea, traitorous blood can take generations to be washed away.

A 1972 law passed by then-ruler Kim il-Sung declared that: "Enemies of class, whoever they are, their seed must be eliminated through three generations."

Having relatives who collaborated with the hated Japanese occupiers or who helped the capitalist South during the war was enough to have someone classified an "enemy of the state".

Tens of thousands of North Koreans have been sent to the gulag for the crimes of their forebears, and many have died in the prison camps.

"I discovered documents in the defence ministry archive in Tokyo which reveal that Kim Jong-un's maternal grandfather Ko Gyon-tek worked at the Hirota Sewing factory. What's significant about that is that it was run by Japan's ministry of war, in other words the Japanese army," Mr Kato told the ABC.

"Collaborators with Japan were categorised as a hostile class, the lowest class in North Korea.

"In many cases they are branded as traitors and sent to political prison camps where many died."

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Collaborator or not, Ko Gyon-tek would escape the gulag - thanks to his beautiful daughter Ko Young-hee, who was born in Japan.

"Ko Young-hee went to North Korea and became a dancer," Mr Kato explained.

"She became a favourite of Kim Jong-il and later his mistress. When Kim's wife died she married him and had three children by him. One of them is Kim Jong-un."

The ABC has seen copies of the documents discovered by Ken Kato - documents which could destroy many myths in North Korea about the Kim dynasty.

"I want Kim Jong-un to realise he's not at the top of North Korea's class system but rather that he's a member of the lowest class - the hostile class," Mr Kato said.

"So he should immediately disband his prison camps where the hostile class are kept, and free his comrades."

It appears that, under his grandfather's own law, Kim Jong-un's veins flow with traitorous blood.

But instead of being branded an enemy of the state, he is the commander-in-chief of a dynastic state based of absolute fear, total control, and unfettered hypocrisy.