Prospero, the exiled Duke of Milan, has always been one of Shakespeare's most mysterious creations. Part mystic, part wizard, he weaves spells and conjures up storms. At the end of The Tempest he utters one of the great speeches in all the Shakespearian canon - “Now my charms are all o'erthrown; and what strength I have's mine own.”

No one has ever been able to say with certainty what, or who, inspired the creation of Prospero, though many of Shakespeare's characters were based on real people and events. Now, however, an amateur historian, rifling through the papers of an eccentric 16th-century Scottish Earl, has uncovered the life of a man he says may have given Shakespeare the idea for the character.

Brian Moffat, a 64-year-old retired policeman from Teviothead, in the Borders, said he stumbled upon the revelation after he and his wife bought an old chest that turned out to be the marriage trunk of Francis Stewart, the Fifth Earl of Bothwell, whose extraordinary antics and rebellious behaviour caused a political and religious scandal.

Mr Moffat decided to investigate carvings of Christian, pagan and satanic symbols on the trunk.

Stewart believed that his cousin, James VI, should invade England to avenge the execution of Mary Queen of Scots. When the King refused to consider it, he turned against him.

Stewart was implicated in plots to kill the King and was rumoured to be heavily involved in witchcraft and sorcery. In 1590 he was said to have dressed as the devil during a witches' sabbath, and cast a spell, summoning up a storm - just as Prospero did - in an attempt to wreck the king's ship. He failed, and James survived to ascend the English throne as well 13 years later. Stewart was imprisoned.

Mr Moffat believes that Shakespeare may have heard the stories of his eccentric behaviour from King James's jester, Archie Armstrong, a high-ranking member of the king's court who is thought to have inspired the character of the fool in King Lear.

“In 1590 Francis Stewart appeared in a pulpit at North Berwick Kirk dressed as the devil and summoned a storm to sink the King's ship,” said Mr Moffat. “That incident is the starting point of The Tempest. There you have an exiled nobleman, who is also a necromancer, who summons up a storm to sink the ruling Duke's ship. The similarities between the accounts and Shakespeare's plot are striking. It is very likely Stewart is the inspiration for Prospero.”

Now Mr Moffat has written a book, Death, Resurrection and the Sword, in which he claims that Stewart's link to Shakespeare's play has been over-looked by scholars because, at the time, Stewart's links to Freemasonry and the occult, along with his “dangerous” political beliefs, caused him to be “written out of history”.

He said: “This is an incredibly important piece of our history. Francis Stewart's castle, Branxholme Castle, still stands. This is Prospero's castle. This is Homecoming year, so we should really be celebrating this important link.”

Like Prospero, Stewart was finally exiled by his political rival, James VI. He was charged with treason for his part in a plot to abduct the King from Holyrood Palace in 1589, and also stood trial in 1591 on charges of witchcraft after the North Berwick Kirk incident. In 1594 he fell out with the King again.

He was finally exiled in 1595 and died penniless in Naples in 1612.

Dr Sarah Carpenter, who lectures on Shakespeare at the University of Edinburgh, said Moffat's theory could not be proved beyond doubt, but a link with Stewart was possible.

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