The Other Prince William: Prince William of Gloucester

Prince William of Gloucester (1941-1972) was a grandson of King George V and a paternal first cousin of Queen Elizabeth II. William's father was Prince Henry, Duke of Gloucester, the third son of George V and Queen Mary. His mother was Alice, Duchess of Gloucester, the third daughter of the 7th Duke of Buccleuch and Lady Margaret Bridgeman. At the time of William's birth, he was fourth in line of succession to the British throne. 

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William remains the most recent descendant of George III to be diagnosed with porphyria, likely hereditary, which is widely believed to be the illness that most likely caused George III's mental breakdown. The prince died in 1972, aged thirty, in an air crash while piloting his plane in a competition.

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Between 1968 and 1970, Prince William of Gloucester was in a relationship with Zsuzsi Starkloff. In 2015, Channel 4 TV released a documentary on the topic of Prince William and Zsuzsi. One may view the film at the link below.