Divers have begun excavating the site where an 18th century vessel foundered off Anglesey in the hope of discovering gold bullion that the King of France sent to aid Bonnie Prince Charlie after the failure of the Jacobite Rebellion.

The team acknowledges that they are basing their hopes on flimsy evidence - a tiny copper disc bearing the royal seal of Mary Queen of Scots and a maritime survey suggesting large underwater metal deposits. But if their theory proves correct, the rewards will be enormous.

Entombed in iron boxes under the seabed on Holy Island close to Porth Dafarch Beach, north Wales, could be 852,000 Louise d’or coins, a priceless cargo in today’s terms.

It would also represent one of the most astonishing finds of its kind in British coastal waters, offering fresh evidence in the story of Charles Edward Stewart’s flight from the English after bloody defeat at the Battle of Culloden in 1745.

A small team of drivers has already begun preparatory work but the main excavation will get underway later this summer with the construction of scaffolding hugging a rugged rock face. This will offer the team of 12 divers a platform from which to explore the inlet where they believe up to two vessels sank in its treacherous waters more than two and a half centuries ago.

Joe McCormack, 65, who runs Maritime Resurgence, the company formed to carry out the dive, told how a seemingly unimportant find by his son, Kevin, a professional diver, 15 years ago led to his current conviction that an “obscene amount of gold” is waiting just beyond reach off the coastline.

A lump of seabed detritus was chipped away to reveal a copper disc or matrix that has been authenticated by the National Museum of Scotland as the seal worn on the signet ring by Mary Queen of Scots at her execution. It was, he believes, to be used to convince the rebellious Scottish aristocracy of their good faith.

Other recovered artefacts, including a miniature cannon used by French master gunners to test the powder and an English gold coin minted in France dated 1746 have added further weight to the theory.

Mr McCormack is convinced that they come from several privateer vessels sent by Louis XV of France to supply or rescue Bonnie Prince Charlie as he languished in hiding.

It is known that the king sent at least two ships, Le Mars and La Bellone, laden with bullion and weapons. These vessels turned tail and limped home to France after an encounter with the British navy. The gold never reached the young pretender.

It is the dive team’s contention that the French king, infuriated by the failure of the expedition, agreed to a covert operation under which two unremarkable captured English vessels were loaded with the precious cargo in harbour and then ordered to sneak up the English coast. In ink black darkness they were sucked onto the coastline off Anglesey and sank.

Mr McCormack said that their surveys suggest that the wooden hulls were smashed on the rocks and carried away leaving the cargo of iron boxes filled with gold coins. They formed a natural dam which, over time, has been covered with boulders and a smooth covering of shingle and stones. A historical land slip has completely submerged one cargo making their work even more difficult, he said.

He was able to quote one historical record drawn from the Palace of Versailles where plaintive questions were asked of the fate of two vessels that left Dunkerque for England. As yet he does not even know the names of the vessels, but he robustly defended his “leap if the imagination”.

He said: “It would be stupid to send the Mary Queen of Scot’s seal up the dukes, who were all committing high treason, as a token of authenticity and then say ’here we are but there is no gold’.

“The one coin that has been found there could have been from any country but it is French dating from 1746. That puts the lid on it. If it looks like a duck and quacks like a duck, then it is a duck. The duck is saying come and get me and enjoy the feast because it will be a feast when we get there”.

Earlier exploratory dives in the 1990s were hampered by the proximity to a steep cliff face but Mr McCormack’s team, now the official salvors in possession of the site, have devised the flight of scaffolding which will allow divers to work at all times of the tide and in all weathers.

Over the summer the divers, working in relatively shallow waters, are scheduled to begin the laborious task of excavating the shingle. Larger boulders will be removed using airlifts. Each step will be recorded by a television documentary crew.

Although they say one minor planning hurdle has yet to be overcome they are confident that the requisite licences and permissions from the local authority, Ynys Mon, and CADW, the historical monuments authority for Wales, have been acquired for work to go ahead. The work, likely to cost in excess of £1 million, could take up to five years to complete.

Mr McCormack said: “It is no exaggeration to suggest that Bonnie Prince Charlie’s wreck could turn out to be the most significant of its kind ever discovered in British coastal waters, both in terms of its historical importance and the value of artefacts which are expected to be recovered”.

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