Greece’s former King Constantine II, whose nine-year reign coincided with one of the most turbulent periods in the country’s political history, has died at a private hospital in Athens, his doctors announced late on Tuesday. He was 82. Constantine, a cousin of British monarch King Charles III, died “of a stroke”. He was admitted to an Athens hospital last week with breathing problems, Greek media reported. When he acceded to the throne as Constantine II at the age of 23 in 1964, the youthful monarch, who had already achieved glory as an Olympic gold medallist in sailing, was hugely popular. By the following year he had squandered much of that support with his active involvement in the machinations that brought down the popularly elected Center Union government of prime minister George Papandreou. The episode, still widely known in Greece as the “apostasy”, or defection from the ruling party of several lawmakers, destabilised the constitutional order and led in 1967 to a military coup. Constantine eventually clashed with the military rulers and was forced into exile. The dictatorship abolished the monarchy in 1973 and when democracy was restored the year after, nearly 70% of Greeks voted for the abolition of the monarchy in a referendum, ending a dynasty begun by Constantine’s Danish-born great-grandfather George I in 1863. In 2008, an opinion poll found fewer than 12% of Greeks favoured a return to a constitutional monarchy. More than 43% blamed Constantine for the coming of the junta. Constantine was born on 2 June 1940 in Athens to Princess Frederica of Hanover and Prince Paul, younger brother of King George II and heir presumptive to the throne. His older sister, Sophia, is the wife of former King Juan Carlos I of Spain. He was also the nephew of Greek-born Prince Philip, the late Duke of Edinburgh and husband of the UK’s late Queen Elizabeth II. The family, which had ruled in Greece from 1863 – apart from a 12-year republican interlude between 1922 and 1935 – was descended from Prince Christian, later Christian IX of Denmark, of the House of Schleswig-Holstein-Sonderburg-Glücksburg branch of the Danish ruling family. When the dictatorship collapsed in July 1974, Constantine was eager to return to Greece but was advised against it by veteran politician Constantine Karamanlis, who returned from exile to head a civilian government. To his final days, Constantine, while accepting that Greece was now a republic, continued to style himself King of Greece and his children as princes and princesses even though Greece no longer recognised titles of nobility. For most of his years in exile, he lived in Hampstead Garden Suburb in north London, and was said to be especially close to his second cousin Charles, now King Charles III. It took Constantine 14 years to return to his country, briefly, to bury his mother, Queen Frederica in 1981, but he eventually moved back permanently. There were continued disputes: in 1994, the then socialist government stripped him of his nationality and expropriated what remained of the royal family’s property. Constantine sued at the European court of human rights and was awarded €12m (£11m) in 2002, a fraction of the €500m he had sought. Constantine travelled with a Danish passport, as a Danish prince. He is survived by his wife, the former Princess Anne-Marie of Denmark, youngest sister of Queen Margrethe II; five children, Alexia, Pavlos, Nikolaos, Theodora and Philippos; and nine grandchildren.