
Constantine II was reportedly admitted to a hospital in Athens last week with breathing problems.
Greek public broadcaster ERT said he died after suffering a stroke.
A close friend of his second cousin King Charles, Constantine was one of William’s six godparents.
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.
Will and Kate speaking with Constantine II during a reception at the Queen’s Jubilee
Prince William with his godfather
The former King had been in hospital
King Constantine II of Greece and his wife Queen Anne-Marie pose for a photo after their wedding in 1964