Prince Charles has arrived at a remote monastery in Greece for a private visit, Buckingham Palace has said.
There are 20 monasteries on Mount Athos
He is to stay at a Greek Orthodox monastery on Mount Athos, which he has visited on several occasions.
Accompanied by two men, believed to be bodyguards, the prince flew to Kavala, 433 miles north-east of Athens, before taking a private yacht to the retreat.
His spokeswoman declined to comment on claims that he would like to convert to the Greek Orthodox church.
The purpose of the stay at the Holy Monastery of Vatopaidi, where monks rise at 4am and follow a daily regimen of chores and prayer, would remain under wraps, insisted the Palace.
"Prince Charles is a regular visitor to Mount Athos but this is very much a private visit," the spokeswoman said. "As such, we will not be revealing any details of the trip."
Mount Athos has been a centre of Eastern Orthodoxy since AD963 and is classed as a semi-autonomous monastic republic.
The Guardian newspaper reported that the frequency of the prince's visits had fuelled speculation among clerics in Greece that he is interested in converting to the religion.
According to legend, the Virgin Mary was blown ashore there on her way to Cyprus and was so overwhelmed by the mountain's beauty that she asked God to give it to her as a present.
Monks on the rugged peninsula, who call it the Garden of the Holy Virgin, live in a self-governing community dedicating themselves to quiet contemplation of the Virgin and God.
It has Russian, Serbian, Bulgarian and Romanian religious houses among its 20 monasteries and the monks are entitled to Greek and European citizenship regardless of their country of origin.
Women - even many female domestic animals - have not been allowed on the mountain since a decree banning women was issued by Byzantine Emperor Constantine Monomachos in 1060.
As well as visiting the Vatopaidi retreat regularly, both Prince Charles and Prince Philip are members of The Friends of Mount Athos, a society formed in 1990 by people with a shared interest in the monasteries of Mount Athos.