Charles, who turns 70 next week, will be the oldest monarch to be crowned when he finally succeeds his 92-year-old mother Queen Elizabeth.

Tom Bower, whose unauthorised Rebel Prince biography of Charles was published earlier this year, said the prince was intelligent, kind and sensitive but also selfish, ungrateful, and a lover of luxury whose stubborn streak could risk the institution itself.

"I think Charles will try his hardest to be a good king," Bower, who describes himself as a committed monarchist, told Reuters. "The question will be how he behaves, whether he abandons a lot of the qualities that were shown in the preceding 20, 30 years.

"I do believe the queen and (her husband) Prince Philip have been thankful to live so long to prevent their son being the monarch because he would have jeopardized it."

Such critical portraits of the prince are not new. Since the public breakdown of his marriage to Princess Diana in the 1990s, his lifestyle and views on issues such as climate change, religion, alternative medicine and architecture have often had unfavorable treatment.

"As a teenager, I remember feeling deeply about this appallingly excessive demolition job being done on every aspect of life," Charles said in a written response to Vanity Fair magazine for an interview published this month.

"In putting my head above the parapet on all these issues, and trying to remind people of their long-term, timeless relevance to our human experience - never mind trying to do something about them - I found myself in conflict with the conventional outlook which, as I discovered, is not exactly the most pleasant situation to find yourself."

Bower, whose biography was based on interviews with 120 people including some who worked closely for the royals, said the prince was committed to issues like the environment but was someone unable to take criticism.

"He's very keen to criticize others but cannot tolerate those who challenge him," Bower said.

"He's a person who is driven, who undoubtedly wants to do good but doesn't understand that the consequences of a lot of his actions cause a lot of trouble and he doesn't like to be told that he might be doing something wrong."

Former aides who have worked closely with Charles say many of the stories in Bower's book are simply not true. The prince himself has dismissed a story that he travels with his own toilet seat.

"I can understand why critics will write ... negatively, but all they're doing is taking a facet of him and making it the most negative possible," one former close aide of many years, who described himself as a big fan of Charles, told Reuters.

"It's not such a contradiction that people have these polar views of him because somewhere in the middle is the real man."

Read also: The man who would be king, eventually: Prince Charles turns 70

Axes to grind

Supporters of the prince say his detractors are often those with axes to grind airing exaggerated grievances.

"That reflects on Charles for causing those people to have a grievance," Bower said. "You don't find people speaking with grievances against the queen."

While the queen was a unifier, he said: "Charles does the opposite. He divides the nation between those who like him and dislike him, he divides his own court, he creates hostility when he be creating harmony and that's his trait."

Bower said Charles had rebelled against his parents, saying the demise of his relationship with Diana and the romance with his second wife Camilla was part of that rebellion.

"He has a view of the world and he wants to impose his view of that world, so in every way he doesn't want to conform to expectations, so that makes him a rebel," he said.

"I think that if he's a rebel king, the monarchy will be in danger and I think that is the great problem we face."

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