Divers believe that they have found one of Britain’s most romantic shipwrecks, reputedly laden with jewels owned by the wealthiest and most notorious actress and opera singer of her day.

When The Nancy was dashed to pieces off the Isles of Scilly in February 1784 the victims included Ann Cargill, a singer whose many affairs and elopements had scandalised London.

Her body was recovered a week after the sinking off the uninhabited island of Rosevear, with an infant – believed to be her illegitimate child – cradled in her arms. But neither The Nancy nor Mrs Cargill’s vast fortune in jewels and cash – amounting to at least £200,000, according to newspapers at the time – was ever found.

Two local divers who claim to have identified the wreck believe that this was because until now searchers have been looking in the wrong place.

Accounts of the sinking told of the ship being “driven into Rosevear”, but the wreck’s finders, Ed Cumming and Todd Stevens, who are also local historians, say that The Nancy actually sank on the islands’ dangerous Western Rocks.

They have located anchors, a cannon and other artefacts from a wreck dating from the correct period nearly half a mile from where The Nancy supposedly went down. They think that the ship’s passengers and crew had taken to a smaller boat that capsized as they tried to land.

All 36 crew, 12 passengers and an infamous prisoner, Sergeant Tooley, drowned. The ghost of Mrs Cargill, The Nancy’s best-known victim, is said to haunt Rosevear.

Mrs Cargill, who was born Ann Brown, was the Charlotte Church of her day. She made her debut at the age of 11 singing the role of Titania in Thomas Arne’s The Fairy Prince at Covent Garden in 1771. She swiftly became a star, taking lead roles until she ran off with the playwright and gunpowder maker Miles Peter Andrews.

Because of her age her father won a court order confining her to his house, which she defied, and in 1776 she was cast in the lead role of Polly in John Gay’s The Beggar’s Opera. Her father attempted to grab her as she made her way to the theatre, but he was restrained by the audience and the performance went ahead.

Three years later she did it again, eloping to Edinburgh with a Mr Cargill who was using the name Doyle to evade his many creditors. After the couple married she returned to London to play MacHeath in a gender-bending version of Gay’s opera.

In both 1779 and 1780 she was the world’s highest-paid actress, earning then the vast sum of £10 a week. At the height of her fame she fell in love again, with Captain John Haldane, known as the unluckiest commander in the British East India Company. He had already lost one ship when he and Mrs Cargill sailed for India.

Soon after their arrival in Bombay his new ship, The Fairford, was destroyed by fire.

Mrs Cargill’s performances caused a sensation in Bombay and Calcutta, earning her – according to one contemporary – “wealth beyond the dreams of avarice” as she was showered with gifts by rich admirers.

She was forced to leave India after the British Prime Minister, William Pitt the Younger, told Parliament that “an actress should not be defiling the pure shores of India”.

After Pitt’s condemnation, Haldane took command of The Nancy, a fast packet carrying mail from Bombay. During the three-month voyage back to England The Nancy was diverted to St Helena in the south Atlantic, where she took on board the leader of a mutiny of the island’s garrison. All perished when The Nancy struck disaster when she was almost in sight of home.

Mr Cumming, a retired analytical chemist, and Mr Stevens have been diving on the wreck for two years and are convinced that it is The Nancy, although they have yet to find definitive proof. Among the clues are fragments of Chinese pottery used only on vessels of the East India Company.

The pair have now “adopted” the wreck through the Nautical Archaeology Society, and any treasure will have to be logged and reported to the Receiver of Wreck at the Maritime and Coastguard Agency.

The wreck is scattered over at least 300 sq metres, making diving on the site difficult and possible only in certain conditions.

Although Mr Cumming and Mr Stevens intend to continue searching for Mrs Cargill’s jewels, they disagree about their chances of finding them. Mr Cumming believes that any treasure was recovered by local people soon after the ship sank, although it was never declared. But Mr Stevens said: “Ed is a pessimist and he thinks it’s gone, but I am an optimist and I think it’s still there.” If any treasure is recovered it will be put on public display in a museum in the Isles of Scilly.

The divers have also written a book about their find, The Ghosts of Rosevear.

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