The youngest son of Luxembourg’s Prince Robert and Princess Julie of Nassau has died from a rare genetic condition at the age of 22.

Prince Frederik died on March 1 from POLG Mitochondrial disease, his father announced in a lengthy statement on The POLG Foundation’s website.

POLG Mitochondrial disease is a genetic disorder that robs the body’s cells of energy, eventually causing multiple organ failure.

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Those with the condition usually have a life expectancy of between three months and 12 years from the onset of symptoms.

Prince Robert called his son a “superhero” in the emotional statement on the charitable organisation’s website.

“It is with a very heavy heart that my wife and I would like to inform you of the passing of our son, The POLG Foundation founder and creative director, Frederik,” he said.

“Last Friday, February 28th, on ‘Rare Disease Day’, our beloved son called us in to his room to speak to him for one last time.

“Frederik found the strength and the courage to say goodbye to each of us in turn.

“After gifting each of us with our farewells — some kind, some wise, some instructive — in true Frederik fashion, he left us collectively with a final long-standing family joke.

“Even in his last moments, his humour, and his boundless compassion, compelled him to leave us with one last laugh ... to cheer us all up.”

"“Frederik’s indomitable lust for life propelled him through the hardest of physical and mental challenges.”"

Prince Robert said one of the young prince’s final questions to his father was asking him if he was proud of him.

“The answer was very easy, and he had heard it oh so many times ... but at this time, he needed reassurance that he had contributed all that he possibly could in his short and beautiful existence and that he could now finally move on,” Prince Robert said.

“Frederik knows that he is my Superhero, as he is to all of our family, and to so very many good friends and now in great part thanks to his POLG Foundation, to so very many people the world over.”

Prince Robert said when Frederik was younger, he believed he was the one child he would “never have to worry about”.

“As his siblings and most anyone who ever met him say, ‘Frederik is the strongest person that we know’,” he said.

“Frederik fought his disease valiantly until the very end. His indomitable lust for life propelled him through the hardest of physical and mental challenges. Our cheery Frederik saw the beauty in everything.”

The young prince was diagnosed at 14 with the hard-to-detect condition.

POLG Mitochondrial disease is thought to affect about 2 per cent of the world’s population.