Prince Frederik of Luxembourg has died at the age of 22

In a message posted onto the PLG Foundation website, he said: "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.

"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 - his brother, Alexander; his sister, Charlotte; me; his three cousins, Charly, Louis, and Donall; his brother-in-law, Mansour; and finally, his Aunt Charlotte and Uncle Mark. He had already spoken all that was in his heart to his extraordinary mother, who had not left his side in 15 years. 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 last question to me, prior to his other remarks was: 'Papa, are you proud of me?'

"He had barely been able to speak for several days, so the clarity of these words was as surprising as the weight of the moment was profound.

"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."

The POLG Foundation website explains that the rare genetic disorder is a genetic mitochondrial disorder that robs the body's cells of energy, in turn causing progressive multiple organ (brain, nerves, liver, intestines, muscles, swallowing and ocular function, etc.) dysfunction and failure.