The former king was also godfather to the daughter of Prince and Princess of Michael of Kent, Lady Gabriella Kingston.
Constantine acceded to the throne at the age of 23 in 1964 and was initially 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 Centre 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 politicians, destabilised the constitutional order and led to a military coup in 1967.
Constantine eventually clashed with the military rulers and was forced into exile.
The dictatorship abolished the monarchy in 1973 and a referendum after democracy was restored in 1974 dashed any hopes Constantine had of reigning again.
Charles was unable to attend Constantine’s funeral in Athens last year because of commitments which included meeting the president of Cyprus. The Princess Royal represented him at the service.
Meanwhile, the King has reportedly sought spiritual advice from his friend Archimandrite Ephraim, Abbot of the Greek Orthodox Vatopedi monastery on Mount Athos.
Abbot Ephraim is said to have told Greek media outlets: “Yes, he has been in contact since the diagnosis and I believe he’ll overcome it. Charles has a spiritual sophistication, a spiritual life.”
The monarch was diagnosed with an undisclosed form of cancer earlier this month and is not carrying out public duties while he has treatment.
