He died at a private hospital in Athens, his doctors announced late on Tuesday. He was 82.
Staff at the private Hygeia Hospital in Athens confirmed that Constantine had died after treatment in an intensive care unit, but had no further details pending an official announcement, The Associated Press reports.
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 medalist 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, destabilized 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 while a referendum after democracy was restored in 1974 dashed any hopes that Constantine had of ever reigning again.
Reduced in the following decades to only fleeting visits to Greece that raised a political and media storm each time, he was able in his waning years to settle again in his home country, when opposing his presence no longer held currency as a badge of vigilant republicanism. With minimal nostalgia for the monarchy in Greece, Constantine became a relatively uncontroversial figure from the past.
Constantine was born on 2 June 1940, in Athens, to Prince Paul, younger brother to King George II and heir presumptive to the throne, and princess Federica of Hanover. His older sister Sophia is the wife of former King Juan Carlos I of Spain. The Greek-born Prince Philip, the late Duke of Edinburgh and husband of the United Kingdom’s late Queen Elizabeth II, was an uncle.
The family, which had ruled in Greece from 1863 apart from a 12-year republican interlude between 1922-1935, was descended from Prince Christian, later Christian IX of Denmark, of the House of Schleswig-Holstein-Sonderburg-Glücksburg branch of the Danish ruling family.
Before Constantine’s first birthday, the royal family was forced to flee Greece during the German invasion in World War II, moving to Alexandria in Egypt, South Africa, and back to Alexandria. King George II returned to Greece in 1946, following a disputed referendum, but died a few months later, making Constantine the heir to King Paul I.