The last monarch of Greece had been hospitalized for several days in a local hospital where he died on Tuesday, Xinhua news agency quoted Greek national broadcaster ERT as saying in a report.
Constantine II became king in 1964, but was forced into exile after a failed counter-coup against the military dictatorship which took over in 1967.
The military junta abolished the Greek monarchy in 1973.
The former king had lived in London for decades and returned to Greece a few years ago following lengthy legal disputes with the state over seized properties.
Constantine was born on June 2, 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-Glcksburg branch of the Danish ruling family.
