He was gay, wildly eccentric and built fairytale castles that today rate as Germany's leading tourist attractions – but more than a century ago "Mad King" Ludwig II of Bavaria was declared insane, deposed and three days later his corpse was found floating in a lake south of Munich.

The real cause of King Ludwig's death has been a mystery ever since his body, together with that of his psychiatrist, was dragged from Lake Starnberg on 13 June, 1886. But the official version, which holds that he committed suicide by drowning, has never been completely refuted.

Now, 111 years after the king's death, new evidence has surfaced which suggests that the builder of Neuschwanstein castle and many other bizarrely romantic architectural follies was murdered. The details are convincing enough to increase calls for the House of Wittelsbach, King Ludwig's family, to allow his body to be exhumed from its tomb in St Michael's Church in Munich to enable a new and conclusive post-mortem examination to be conducted.

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The most intriguing new material to support the murder theory has come from a 60-year-old Munich banker called Detlev Utermöhle. In a sworn affidavit issued earlier this month, Mr Utermöhle recalled a scene from his childhood which he insists he remembers vividly.

As a 10-year-old, he and his mother were invited for afternoon coffee and cakes by a Countess Josephine von Wrba-Kaunitz, who looked after some of the Wittelsbach family's assets. Mr Utermöhle recalled how the countess gathered her guests, telling them in a hushed tone: "Now you will find out the truth about Ludwig's death without his family knowing. I will show you all the coat he wore on the day he died." The countess opened a chest and pulled out a grey Loden coat. Mr Utermöhle insists in his statement that he saw "two bullet holes in its back" and says his mother, who has since died, left him a written account of what they saw.

Unfortunately for Mr Utermöhle, the king's coat was lost after a fire at Countess Wrba-Kaunitz's home in 1973 in which both she and her husband perished. However his claims were supported this week by Siegfried Wichmann, a Bavarian art historian and specialist in 19th-century painting, who published a hitherto unseen photograph of a portrait of the king painted only hours after his death.

The portrait shows what Mr Wichmann says is blood oozing from the corner of Ludwig's mouth. "King Ludwig cannot have drowned. This is blood from the lungs and there is no water in it," Mr Wichmann insisted on Wednesday.

The official version holds that the Bavarian government was driven to depose the reclusive Ludwig because he was squandering vast sums of money on bizarre building projects that were driving his kingdom to ruin.

Bernhard von Gudden, his psychiatrist, diagnosed him as suffering from "paranoia" – a condition which today would be classified as schizophrenia. Ludwig was deprived of his crown and, according to the official version, he reacted by drowning himself in Lake Starnberg in a fit of paranoid pique.

Murder theorists counter with recent medical evidence which suggests that the king was, in fact, suffering from a form of meningitis and was far from insane. They say fishermen reported hearing shots at the time of Ludwig's death and claim that his opponents in the Bavarian government hired assassins to kill him as he was trying to flee across the lake. They say that Von Gudden, who was also found dead in the lake, was shot because he was a witness.

To date, the Wittelsbach family has dismissed all murder theories and refused point blank to have the king's body exhumed. The latest attempt to persuade them to change their minds comes from the Berlin historian and author, Peter Glowasz, who wants to employ Swiss scientists to examine the corpse by giving it a computer tomography. He insists that while the procedure would not touch the body, it would show up any gunshot wounds.