The man believed to be Kim Jong-nam
The man believed to be Kim Jong-nam Reuters

North Korea's 'Great Successor' Kim Jong-Un has several critics, but a new book reveals that his brother, the late leader Kim Jong-il's eldest son, might be his most outspoken.

The brothers, Kim Jong-un and the elder Kim Jong-nam, have shown little brotherly love, according to a new book by Japanese writer Yoji Gomi.

"My Father, Kim Jong-il, and Me" exposes emails and interviews between Gomi and the older brother after they met in 2004 in Beijing.

Known for his playboy lifestyle, Jong-nam, once the regime's heir apparent, angered his father after he tried to enter Japan with forged documents a decade ago. It was alleged that he wanted to visit Disneyland.

Since then he has been sharing his time between China's gambling and casino enclave of Macau and mainland China.

The new book shows him as a vocal critic about the rest of his family. He openly questions his father's military rule, casts doubt on his little brother's new position and warns of the need for economic reforms, according to Gomi.

"My father governed the country with the backing of the military but the power of the military has become too strong," he says in the book. "If the succession ends in failure, the military will wield the real power for sure."

While both sons were educated in Switzerland they were kept apart and now Jong-nan fears that his younger brother's inexperience might led the country into further economic chaos.

"Anyone with normal thinking would find it difficult to tolerate three generations of hereditary succession," he reportedly said in an email to Gomi.

"I question how a young heir with two years [of training as a successor] would be able to inherit ... absolute power," he said.

"It is likely that the existing power elites will succeed my father by keeping the young successor as a symbol."

Kim Jong-un's eldest brother also warns that if the economic reforms needed to avert complete economic collapse are applied, the authoritarian regime will fall.

"It is obvious that [the] economy will collapse without reforms,but the reforms will lead to a crisis of the collapse of the regime," Jong-nam said in an interview before his father died on December 17.