The battle for gay rights may have been fought more than two centuries before the UK legalisation of homosexuality.
Dr Hal Gladfelder discovered the document by chance
The 18th Century writings of Thomas Cannon, believed to be one of the first gay activists, have been found by a University of Manchester academic.
They were contained in a handwritten scroll indicting the printer of his 1749 work "Ancient And Modern Pederasty Investigated And Exemplified".
The book was banned but the scroll has long, previously unheard, extracts.
Dr Hal Gladfelder found the parchment among a box of uncatalogued documents from 1750 while doing research at the National Archives in Kew.
The indictment suggests the book was an anthology of stories and philosophical texts in defence of male homosexuality.
One story deals with cross-dressing while others are translations of Greek and Latin homo-erotic texts.
One of the extracts reads: "Unnatural desire is a contradiction in terms; downright nonsense.
"Desire is an amatory impulse of the inmost human parts."
Dr Gladfelder, from the School of Arts, Histories and Cultures, said: "This must be the first substantial treatment of homosexuality ever in English.
"The only other discussions of homosexuality were contained in violently moralistic and homophobic attacks or in trial reports for the crime of sodomy up to and beyond 1750."
Sodomy was a capital offence punishable by death until 1861 and homosexuality was banned until 1967.
Dr Gladfelder said Cannon fled to Europe to avoid punishment and no copies of the book itself survive.
"It's a fair assumption that Cannon was writing for a gay subculture at the time - which has largely remained hidden," he added.
"Though he lived in anonymity - possibly because of the notoriety of his pamphlet - I certainly regard him as a martyr.
"I think what happened to Cannon paved the way for 200 years of homophobic repression," he added.