A long-lost film by Hollywood legend Orson Welles has been discovered in a warehouse in Italy, and will finally premier this Autumn – 75 years after it was made.

Welles, who leapt to stardom in 1941 as the director and star of Citizen Kane, stopped work on the comedy Too Much Johnson three years earlier.

The 35mm nitrate work print was found in a warehouse by the staff of the arthouse cinema Cinemazero in Pordenone, north-east Italy earlier this year.

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Until the discovery, it was thought the only known copy was lost in a fire that destroyed Welles’s home near Madrid in 1970. How it ended up in Italy “is still a mystery,” said Kellie Fraver, a spokeswoman for the George Eastman House museum of film and photography in Rochester, New York state, which announced its restoration of the film on Wednesday.

The actor-director had intended to show the film as part of the stage adaptation of an 1894 Willam Gillette play. There were plans for the Mercury Theatre, which Welles co-founded, to show the film in three short sections as prologues to each act of the play, according to the US film cinema weekly Variety.

Welles wanted the three-part slapstick comedy to be screened with music and special effects and even got to the point of editing it, but failed to complete the project. It has been suggested that Welles tussled with Paramount Pictures, which owned the film rights to Too Much Johnson, for artistic control over the project. It starred Joseph Cotten, who would later appear in the Welles masterpieces, Citizen Kane and The Magnificent Ambersons.

“This is by far the most important film restoration by George Eastman House in a very long time,” Paolo Cherchi Usai, who supervised the restoration, told Variety.

All of the reels, said Usai, were in “relatively good shape. But one was badly decomposed, and we initially thought it was too late to save the image.”

But Haghefilm Digitaal, a preservation lab in the Netherlands, was able to save 96 per cent of the footage. “I’d call it a masterpiece of craftsmanship,” said Usai. “Holding in one’s hands the very same print that had been personally edited by Orson Welles 75 years ago provokes an emotion that’s impossible to describe.”

The actor-writer Simon Callow who is currently working on the third volume of his biography of the film-maker, said:”The discovery of the long-lost footage from Orson Welles’ out-of-town production of Too Much Johnson is thrilling; a very significant piece of the jigsaw of Welles’ art.”

The restored film will premiere on 9 October at Pordenone, at Italy’s silent film festival. Its US premiere will be on 16 October.

Welles, who won the best screenplay Oscar for Citizen Kane, was also famed for his 1938 radio drama The War of the Worlds that scared many listeners into believing the Earth was really under attack from Martians. He died in 1985 at the age of 70.