IOC President Thomas Bach led the tributes to Constantine. 

"In King Constantine we have lost a great friend of sport," he said.

"He was a fellow Olympic champion and whenever we met, we shared our passion about sport and discussed our love of Olympism and the life of an athlete.

"He was always interested in the development of the Olympic Movement, and our conversations were very enriching.

"I will greatly miss these always friendly meetings. Our thoughts are with Queen Anne-Marie and with the entire family,"

Constantine was admitted to the private Hygeia Hospital in Athens with breathing difficulties last week, and is reported locally to have died after suffering a stroke.

He had reigned as King of Greece from 1964 to 1967, but went into exile after a coup and had lived for many years in London.

Constantine was a nephew of the late Prince Philip, Duke of Edinburgh and a godparent to Prince William.

Constantine had been elected President of the Hellenic Olympic Committee when he was only 15-years-old.

As a young man, he practised karate and became a black belt in the sport and attended the formative sessions of the International Olympic Academy in Ancient Olympia.

Constantine also took up sailing whilst still a Prince and was coached by the legendary Danish four-time Olympic gold medallist Paul Elvstrom.

He was selected for the Greek sailing team which competed in the sailing regatta at the 1960 Olympics, held in the Bay of Naples.

Earlier, he had participated in the Flame Lighting Ceremony at Ancient Olympia when he handed the Torch to the first runner.

He carried the Greek flag at the Opening Ceremony of Rome 1960.

Constantine's boat Nirefs won gold in the dragon class at the same Games.

His sister Princess Sophia had been reserve for the crew and Constantine arranged for a necklace to be made for her in the shape of the Olympic rings to acknowledge her contribution to the team.

Constantine remained close to the sport of sailing for the rest of his life and became World Sailing's President of Honour in 1994.

In 2010, he was awarded the Beppe Croce Trophy by World Sailing in honour of his outstanding voluntary contributions to the sport.

"World Sailing is deeply saddened by the passing of King Constantine," a statement said today.

"Our thoughts are with his family at this time, and we join the rest of the sailing world in mourning his passing while also celebrating his contribution to the sport."

Constantine was elected as a member of the IOC in 1963, a year before acceding to the throne of Greece on the death of his father King Paul.

Constantine was forced into exile in 1967 when a military junta assumed control of Greece, but officially remained King until 1974 when a referendum abolished the Greek monarchy.

He also resigned his IOC membership, but was accepted as an Honorary member.

He eventually returned to live in Greece from 2010 until his death.

Constantine was seen in a wheelchair in his most recent public appearances.