Did the 'Pirate King' become a SPY? Unearthed letter is claimed to solve mystery of what happened to notorious buccaneer Captain Henry Avery who vanished without a trace after pillaging £90m of Mughal coins

Henry Avery and his crew helped plunder an armed Mughal trading vessel

A 'Pirate King' who pillaged £90million worth of Mughal coins vanished without a trace to become a British spy, experts now believe.

Notorious pirate Captain Henry Avery committed the most lucrative act of piracy the world had ever seen before disappearing in 1696 to never be seen again.

His disappearance had been a mystery for more than 300 years but researchers now believe an unearthed letter written by Captain Avery himself in December 1700 has solved the case.

Dr Sean Kingsley, a marine archaeologist and pirate expert, says the letter shows that Captain Avery had taken a royal pardon to work for her Majesty's Secret Service.

A year prior to his disappearance, the 150 crewmen of Avery’s 46-gun ship the Fancy pillaged £600,000 of gold, silver and jewels - worth £90 million today - from an armed trading ship of the Mughal empire.

The story of one of Britain's most fabled men has now been revived by Dr Kingsley in a book he has co-authored called - The Pirate King: The Strange Adventures of Henry Avery and the Birth of the Golden Age of Piracy.

Notorious pirate Captain Henry Avery (portrayed as an engraving) committed the most lucrative act of piracy the world had ever seen before disappearing in 1696 to never be seen again

Experts believe an unearthed letter (pictured) written by Captain Avery himself in December 1700 has solved the 300-year-old case of his disappearance

'Pirates weren't good guys, but Avery is misunderstood,' says Kingsley. 'He's seen as a thug who stole an English frigate off Spain in May 1694 and turned pirate out of greed. That's not true.'

'Avery lives on in the public imagination because he was larger than life. Not only did he strike it big, he escaped the authorities with his head intact.'

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Before turning to piracy, Avery began his life upon the sea fighting pirates of the Caribbean as a sailor working for the Spanish shipping expedition.

But this all changed when his Spanish employers refused to pay him and 80 of his other crew members, leading them to steal a ship in May 1694 and take to the high seas.

The following year, Captain Avery and his crew plundered an armed trading ship belonging to Aurangzeb, the ­Mughal Emperor in India, which ­contained treasures worth £600,000 — £90million today.

It was the biggest act of piracy ever known and England put a £1,000 (£120,000 today) bounty on Avery's head to spark the world's first global manhunt.

Some versions of the story also suggest, grimly, that Captain Avery himself found 'something more pleasing than jewels' onboard the vessel — often said to be the daughter, granddaughter or another relative of emperor Aurangzeb.

Captain Avery and his crew initially took their ill-gotten gains to Bourbon (now Réunion), before making way to the island of New Providence in the Bahamas.

Yet news of the bounty placed on their heads soon caught up with them — and what happened to the pirate and many of his crew after they fled has been a mystery ever since.

Avery's fate had puzzled centuries of shipwreck hunters until 1978 when a misfiled letter was found in the Scottish Records Office by writer Zélide ­Cowan, whose husband, Rex, is Kingsley's co-author and pirate enthusiast.

Mr Cowan was unable to decipher the coded letter until he met Kingsley decades later.

Captain Avery (portrayed as an engraving on the left) and his crew plundered an armed trading ship belonging to Aurangzeb, the ­Mughal Emperor in India, which ­contained treasures worth £600,000 — £90million today

In September 7, 1695, Captain Avery's ship, the Fancy, engaged the Ganj-i-Sawai, which was owned by one of the world's most-powerful men, the Mughal emperor Aurangzeb. (Pictured: a 19th century artowrk depicting the battle between the two vessels)

Some versions of the story suggest, grimly, that Captain Every himself found 'something more pleasing than jewels' onboard the vessel — often said to be the daughter, granddaughter or another relative of emperor Aurangzeb (Pictured: a 20th Century illustration depicting Captain Every's encounter with the Emperor's granddaughter)

Dr Kingsley is convinced the letter is a genuine and appears to show that Captain Avery was working to prevent a 'Catholic' invasion from France.

Speaking to the Times, Dr Kingsley said: 'There were fake pirate letters that people were pitching around in the 18th century to sell as a get rich quick scheme, but they are really dodgy and easy to identify — they say things like, 'By this rock walk three steps and go left etc'.

'In this, when he wants to conceal his meaning he uses letters and numbers in a code, and this is ­exactly what spies and ­ambassadors were using. It's also really unlikely that anyone would have the intelligence and information to forge it.'

The pair of authors also believe the document also details a list of treasures that are still waiting to be found.

'The mystery lives on today,' says Cowan. 'At the end of the 'Avery the Pirate' letter is the most astonishing list of treasure that he kept as his share of the booty looted off India. It's the stuff of dreams.'

'Part of it, so the legend goes, is still out there, somewhere buried away just waiting to be found.'