Scottish warlock was inspiration for Mr Hyde

To the great and good of 17th-century Edinburgh, Major Thomas Weir was the epitome of puritanical respectability. An esteemed preacher who railed against sin from his pulpit in the city's West Bow thoroughfare, he and his sister Jean were considered so devout they were known locally as the "Bowhead Saints".

So it came as something of a surprise to his devoted faithful when the Major confessed, at the age of 70, to leading a darker life as a warlock behind a string of horrendous crimes including bestiality, incest, black magic and necromancy.

His trial and subsequent execution for witchcraft in 1670 has gone down in the annals of Edinburgh's folklore. But it appears Major Weir boasts an even more formidable legacy: it was his bizarre, schizophrenic life that Robert Louis Stevenson used as his inspiration for his most infamous of literary creations - Dr Jekyll and Mr Hyde.

According to a BBC documentary, Ian Rankin Investigates: Dr Jekyll, the spilt personality of Major Weir both fascinated and terrified a young Stevenson who was haunted by the ghost stories his nanny, known as "Cummy", would tell him when he was little. "What made Cummy's bedtime stories for young Louis so terrifying, was that they really happened - just outside his bedroom window on the haunted streets of Edinburgh," said the documentary's presenter, the crime writer Ian Rankin.

Stevenson's novella, The Strange Case of Dr Jekyll and Mr Hyde, stunned Victorian London when it was published in 1886 and became an instant best-seller.

Rankin believes that the incredible duplicity of Major Weir, who for decades was able to convince his peers that he was a man beyond sin only to be condemned to death as a follower of the devil provided a perfect muse for the character of Dr Jekyll. Described by Stevenson as a "tall black man" with a "grim countenance and a big nose", Major Weir was the Scottish equivalent of a holy warrior. Throughout the Bishops' Wars, which pitted Scotland's clergy againss Charles I, he fought on behalf of Scotland's strictest religious sect, the Covenators, and, by the time he confessed, was regarded as the most pious man in Edinburgh.

Sentenced to death by strangulation and burning it is said that while tied to the stake the former preacher who had spent lifetime telling his flock to ask forgiveness refused to repent. "Let me alone," he said as the executioner tried one last time to make him pray. "I will not. I have lived as a Beast and I must die as a Beast."

Rankin also believes that the city of Edinburgh itself may have been just as much an inspiration for Dr Jekyll and Mr Hyde as the ghost stories of Major Weir. Stevenson would have been aware of the way the city's elite would often lead duplicitous lives, extolling the virtues of Victorian moralism while drinking and whoring in the city's poorer old quarters.

"The city itself has a split-personality," said Rankin. "Stevenson himself lived a double life enjoying the kind of company that would have appalled his upright and god-fearing parents. The city's haunted and violent past is all around you; it's impossible to escape."