Greece’s last king before the monarchy was abolished, Constantine II, died on Tuesday in Athens, aged 82.

Constantine had been taken to a private hospital in the Greek capital last week after experiencing breathing problems and had been treated in an intensive care unit.

A descendant of Denmark’s royal Glücksburg family, Constantine was a second cousin of Britain’s King Charles III and a godfather to William, prince of Wales.

He acceded the throne at the age of 23 in 1964, just after winning an Olympic gold medal in sailing. But he soon became unpopular, as he clashed with the elected government of George Papandreou. The government fell in 1965 during a political crisis known as Apostasia or the “royal coup,” and Constantine then appointed successive governments with little popular support. This period of political instability ultimately led to the establishment of a military junta in 1967.

Constantine swore in the colonels of the junta and took a photo with them. But later on, he supported the overthrow of the junta, which failed, prompting him to leave Greece.

For many years, he and his wife Anne-Marie lived in Hampstead Garden Suburb, London.

His title as king was officially abolished in a 1974 referendum after the fall of the junta, when Greek voters confirmed the abolition of the monarchy and the establishment of the Third Hellenic Republic.

Constantine traveled with a Danish passport as a Danish prince. He lost his Greek nationality in 1994 after refusing to accept a passport with the last name of Glücksburg, arguing his name was “Constantine of Greece.”

Until his dying moments, Constantine continued to call himself the “king of Greece” and his children “princes” and “princesses,” despite the fact Greece doesn’t recognize titles of nobility.