On 10 June Prince Philip, the Duke of Edinburgh, will celebrate his 89th birthday - not at Buckingham Palace, but in a remote village in the wilds of Melanesia, if an obscure prophecy comes true.

For more than three decades, some of the tribal clans in southwestern Tanna, one of Vanuatu's larger islands, have - strange though it seems - revered the Queen of England's husband. They believe he is the descendant of one of their ancestral spirits, and that he will return to live among them. Now that day, they are convinced, is drawing near.

"He made a promise that in 2010, on his birthday, he will arrive in Tanna," says Siko Nathuan, a tribal leader in the village of Yaohnanen, situated on the island's upper slopes. "I've read it in a document somewhere. We know he is a very old man, but when he comes here, he is going to be young again, and so will everyone else on the island."

Prince Philip's last visit to Vanuatu was in 1974, when he arrived with the Queen on the Royal yacht Britannia. At that time the country, with its 83 populated islands, was still the Anglo-French territory of the New Hebrides. The royal couple did not alight on Tanna, but news of their visit reached its shores and was woven, it seems, into an ancient story that had been awaiting an ending.

According to the story, a dark-skinned spirit figure emerged from a still active volcano on Tanna, holding leaves in his right hand. They indicated mastery over nature. Then a light-skinned figure came out of the volcano clutching a hammer, or a chisel, or a saw. That represented mastery over technology.

"That figure disappeared overseas and maybe was the ancestor of all the white races," says Kirk Huffman, an anthropologist who has studied Vanuatu since the early 1970s. "The clans have been thinking: 'Where has that guy gone and who are his descendants?"'

Then the Duke of Edinburgh appeared, resplendent in a white naval uniform. The belief that he must be the spirit's descendant hardened when the tribespeople learnt that he was not from Australia or England or France or the United States.

"They've heard about these islands, and if he's not from there, there's only one place he can be from: Tanna," says Huffman.

In Yaohnanen, which lies at the top of a steep, rutted track, Nathuan - whose grandfather, Jack Naiva, possibly met Prince Philip - shows the way to a bamboo hut with a thatched roof and dirt floor where the Prince will live when he returns to Tanna.

"I know that in England he has a palace and servants," says Nathuan. "But here he will just live simply, like us."

For now, the hut - which, like other dwellings in Yaohnanen, has no electricity or running water - serves as a shrine. Next to it is a carefully tended garden. From within, the locals produce newspaper cuttings - one, from the London Evening Standard in 2007, is headlined "Prince Philip Health Fears" - and three signed portraits of the Duke, all sent out by Buckingham Palace.

The earliest photograph, a black and white print now damaged by mildew, was delivered by the British Resident Commissioner, J. S. Champion, in 1978, two years before Vanuatu became independent. A decade later, a picture of the Prince grasping a ceremonial pig-killing stick, a gift from the villagers, was sent out, while the most recent framed photo arrived in 2000.

Nathuan believes he is related to the Duke through his grandfather, since Naiva - or Chief Jack, as he was known - was said to be descended from the dark-skinned spirit. Naiva died in 2008, and is buried on the outskirts of the village. A photograph of him holding a picture of the royal couple is embedded in his headstone.

"One of the grandfathers has already died, so we want the other grandfather to come home," says Nathuan, seated beneath a giant banyan tree in Yaohnanen, where men in nambas (penis sheaths) hunt wild pigs with bamboo bows and arrows.

Some describe this curious movement as a "cargo cult" - one of scores that sprang up on Tanna and all over Melanesia following the arrival of Westerners. But Huffman says the term is a misnomer. "I would call it a visionary movement," he says. "The Prince Philip thing is just one aspect of an incredibly sophisticated and very profound culture."

Elsewhere on Tanna, certain clans venerate a shadowy spirit figure known as John Frum. That movement, which emerged in the 1930s, was a reaction against Presbyterian missionaries who had tried to ban traditional customs and beliefs. Huffman believes the Prince Philip movement may have been a counter-reaction to John Frum. "Some of its founders were linked with John Frum very early on, but they thought it had gone wrong," he says.

"They wanted a purer attachment to kastom (the traditional way), and they needed a supporter, a figure from outside. These cultures are taking elements of the outside world and slotting them into their own belief systems in a way that's useful to them."

According to various stories that have become tangled over time, the people of Tanna presented the Duke with a pig, or vice versa - a highly significant gesture in Vanuatu. It is also said that when he came ashore in the capital, Port Vila, he only had time to shake hands with dignitaries from Tanna.

In the 1970s Huffman wrote an explanation of the movement - apparently followed in Yaohnanen and a cluster of neighbouring villages - for British colonial authorities. On his return, he said, the Prince was expected to bring the gift of eternal youth.

He wrote: "At the very moment that he sets foot ashore, mature kava plants will sprout all over the island; all the old people will shed their skins like snakes and become young again; there will no more sickness and no more death ... a man will be able to take any woman he wants."

In Yaohnanen, the locals plan to welcome the Duke with a feast, traditional dancing and kava drinking. "Every year, on June 10, we celebrate his birthday," says Nathuan. "We get together and talk about him. We feel happy. We can feel his presence. Now we're waiting for him to honour his promise to come back in 2010."

At the village school, children are taught about the spirit figure who lives in England. Nathuan says: "When we plant something in the garden, we have to respect it. It's the same with Prince Philip, and when he comes from England, we will respect him like a King of Tanna."

He adds: "Can you tell Prince Philip we are waiting for him? We are his family and we really want him to come home."