Sherlock Holmes would never have stood for this: newly discovered documents show that the Staffordshire police fabricated evidence to try to discredit Arthur Conan Doyle’s investigation into the curious case of George Edalji, a Birmingham solicitor accused of maiming horses and sending poison-pen letters at the turn of the 20th century.

In a printed report discovered in a collection of letters, chief constable of Staffordshire police, GA Anson, admits attempting to discredit Doyle by creating an elaborate ruse. This involved him fabricating a letter to the writer signed “A Nark, London” and setting up various informers to make Conan Doyle believe the letter had been sent by one Royden Sharp, the man the Sherlock Holmes creator had tapped for the so-called “Great Wyrley Outrages”.

“This letter is very well known to writers on the subject, and this document proves the letter was sent by Anson to Conan Doyle to try and discredit him,” said Sarah Lindberg of Bonhams, which today put the report up for auction alongside 30 autographed letters from Doyle about the case, 24 of which are to Anson. “The new evidence was a total fabrication planted by Anson, and all part of an elaborate ruse involving Sharp.”

Julian Barnes, whose Booker-shortlisted novel Arthur and George retells the Edalji story, said he “knew Doyle had received a threatening letter in London, and he drew the obvious conclusion that it was one of the bad guys who was trying to warn him off”.

“The notion that it was actually the chief constable is quite discombobulating,” said Barnes, whose novel has been adapted by ITV into a series starring Martin Clunes as Conan Doyle and Arsher Ali as Edalji.

Facebook Twitter Pinterest Martin Clunes as Arthur Conan Doyle in ITV’s Arthur and George. Photograph: Neil Genower/ITV

The twists and turns of Edalji’s case were complicated enough as it was. Son of an Indian father and English mother, the solicitor had been given seven years’ hard labour in 1903 for animal mutilation and anonymous letter writing. When he was released after three years of his sentence, he asked for help from Conan Doyle to prove his innocence.

The novelist believed Edalji, and was convinced that “colour was involved [in his conviction],” said Barnes, adding that “we would now use ‘institutional racism’ as the way to describe the Staffordshire police constabulary.” Conan Doyle bombarded Anson with almost daily letters between August and October 1907, providing new forensic evidence and offering alternative suspects, petitioning the Home Office, and speaking out publically about the conviction – no small thing, from the creator of the world’s most famous sleuth.

“You could see [Anson] would have been incredibly irritated by this case, which had been long solved,” said Barnes. “But to go as far as fabricating evidence ... He was obviously a very self-confident man, which is partly why he and Doyle fell out so badly. Despite being two British gentlemen, they ended up locking horns like two mad, rutting stags.”

The letters, from a private collection that Bonhams put up for auction on 18 March, reveal the breakdown in the relationship, and show Conan Doyle’s increasing frustration with the situation: “I never thought my case was good enough for a prosecution, but … to say there is ‘absolutely nothing’ against a man who exhibited a weapon and said it was the sort of one which did the outrages is a statement which makes me feel rather hopeless about the use of getting evidence.”

He would later write of his regret that, “to the deep disgrace of the British Administration”, he was unable to secure compensation for Edalji, telling Anson: “Your letter is a series of inaccuracies mixed up with a good deal of rudeness.” Conan Doyle eventually demanded through his solicitors that Anson should not contact him again unless through legal channels.

Anson’s growing anger is also revealed. In one note, he writes: “Is CD mad?” In another, he has it that “this matter is a personal one between Sir A Doyle and myself”.

In an appendix attached to his printed memo on the case, Anson reveals his fabrication of evidence, writing of the author’s reaction: “It was on ‘evidence’ and ‘proof’ such as he obtained in the above described instance that the great Sherlock Holmes based his accusations.”

“He’s implying that [Conan Doyle] is an amateur sleuth,” said Lindberg, adding that Anson appeared “rather proud” of his Nark letter. Anson wrote: “There is a vein of humour running through it which I fear was quite lost on the somewhat obtuse recipient.”

Conan Doyle, however, “did afterwards get an inkling that he had been taken in”, according to Lindberg, writing to Anson in one of the letters: “There is nothing wonderful in this … It in no way destroys my case, as you appear to imagine.”

The Sherlock Holmes creator remained convinced of Edalji’s innocence, as is Barnes today. In 1934, a labourer, Enoch Knowles, confessed to the letter writing, and was imprisoned, but the mutilator of horses remains unknown.

Barnes, meanwhile, has no plans to revisit Arthur and George in the wake of the new evidence, but he may include the revelation in an afterword for the next edition. “I don’t think it changes the substantive outlines of the case, but it’s an interesting detail which tells us a lot about Anson,” said the Booker-winning novelist. “I would have put it in the book if I’d known about it. It’s further proof of the extent to which Anson was entrenched in his position … And it makes me wonder what else he did that he didn’t put in the report.”