In e-mails and interviews with a journalist, Kim Jong Nam, who lives in exile, says that he fell out of favour with his father after pushing him to enact economic and political reforms.

In the most intriguing revelations of all, he admits that he is “under the protection” of the Chinese Government, strengthening speculation that Beijing is keeping him in reserve as a potential future leader of North Korea.

“The Kim Jong Un regime will not last long,” Mr Kim told Yoji Gomi, of the Japanese newspaper Tokyo Shimbun. “Without reforms, North Korea will collapse, and [even if] such changes take place, the regime will collapse. I think we will see valuable time lost as the regime sits idle fretting over whether it should pursue reforms or stick to the present political structure.

“I’m concerned how Jong Un, who merely resembles my grandfather [North Korea’s founding president, Kim Il Sung], will be able to satisfy the needs of North Koreans. Kim Jong Un is still just a nominal figure and the members of the power elite will be the ones in actual power.”

The remarks, whose publication was authorised by Mr Kim, appear to represent a calculated attempt to undermine the new regime by a man who once had hopes of leading it.

“I feel that he has some jealousy about Kim Jong Un because his father, Kim Jong Il, selected him as the next leader,” said Mr Gomi, who encountered Mr Kim by chance in Beijing airport in 2004 and has exchanged almost 100 mails with him since then and interviewed him over seven hours.

He told Voice of America: “Sometimes he said to me that Kim Jong Un has no vision to rule North Korea, how to enrich North Korean people. He said to me many times he has no intention to go back to North Korea. But many people, including North Korean people and Chinese people, expect him to go back to North Korea as a kind of leader.”

Ever since his birth, Kim Jong Nam has been something of an embarrassment to North Korea. He is the son of Sung Hae Rim, a North Korean film star who was Kim Jong Il’s mistress, and who died in Moscow in 2002 – his given name means “loyal man”. As an illegitimate child, his existence was kept a secret for years, and he led an isolated childhood, lavished with toys, foreign clothes and electronic gadgets.

He was educated at a boarding school in Switzerland, and spent his holidays shopping and attending parties in Geneva and Paris. In 2001 he suffered public humiliation when he was deported from Japan for travelling on a false passport of the Dominican Republic. It turned out that he had been visiting Tokyo Disneyland with his wife, along with a young son and secretary.

He told Mr Gomi it was routine for members of the North Korean elite to travel on fake documents. “I went to Japan many times to go to famous hotels and restaurants in Tokyo,” he said. “Jong Un also went to Japan with a fake Brazilian passport.”

Many assumed that it was the Disneyland fiasco that caused his father to pass over Jong Nam, now 40, in favour of his youngest half-brother, Jong Un, who is believed to be 28 or 29. But he told Mr Gomi that the reason was his insistence on reform of North Korea’s economic and diplomatic policies.

“After I went back to North Korea following my education in Switzerland, I grew further apart from my father because I insisted on reform and market-opening and was eventually viewed with suspicion,” he recalls in Mr Gomi’s new book My Father, Kim Jong Il, and I: Kim Jong Nam’s Exclusive Confession, published in Japanese this week.

“My father felt very lonely after sending me to study abroad. Then my half brothers ... were born and his adoration moved on to them. And when he felt that I’d turn into a capitalist after living abroad for years, he cut short the overseas education of my brothers and sister.

“I told him honestly how the international community was concerned about the nuclear tests and missile launches and I am asking him to train my brother [Kim Jong Un] well in order to ensure a good life for the people.”

Surprisingly, Jong Nam says that he has never met Jong Un, although he does know his other half-brother, Jong Chul.

He says that in the past his father did not want to pass the leadership on to one of his sons, but changed his mind because it seemed to be the best way to ensure a smooth transfer of power.

“The dynastic succession is a joke to the outside world,” he tells Mr Gomi. “Rather than welcoming the hereditary succession, China is merely acknowledging it for the sake of maintaining stability.”

He added: “The Chinese Government is protecting me, but it is also monitoring me too. It’s my inevitable fate. If you can’t avoid it, it’s better to enjoy it.

“Because I was educated in the West, I was able to enjoy freedom from early age, and I still love being free. The reason I visit Macau so often is because it’s the most free and liberal place near China, where my family lives.”