He said he had not heard from Kim for a decade, but that they used to watch kung-fu movies together, play computer games, and eat the food cooked up by Kim’s private chef.

“He was a good friend. He was very quiet. He was a nice guy. We weren’t the dimmest kids in class but neither were we the cleverest.

“We were always in the second tier. He left without getting any exam results at all. He was much more interested in football and basketball than in lessons.”…

A US Congressional staff member told The Sunday Telegraph that intelligence analysts have speculated, in closed-door briefings, that the younger Kim may suffer from a personality disorder. He is thought to have a “violent streak”.

Russian sources also confirmed that Kim Jong-un had undergone a drastic change since being picked as his father’s successor, from a quiet and shy child into a tough, rude and even violent man. During one reception in his country home last year, he is said to have humiliated and kicked his waiters while drunk.

He is said already to show symptoms of diabetes and heart disease, suggesting that he may share some of his father’s ill health.