Joiner Ross McMillan discovered the ancient movie hidden behind a partition wall in a projection room in the Grosvenor Cinema.
The Ashton Lane landmark is undergoing a major refurbishment by Stefan King’s G1 Group.
And the tradesman almost threw out the canister while working at The Loft, which is being renamed the Grosvenor Café & Cinema.
The dusty old reel, which dates back to the 1930s, is thought to have been missing from US film production company RKO’s library vaults for more than 70 years.
And the workman originally thought he had unearthed hidden treasure before realising it was a dusty old film reel.
The movie discovered was the original 1933 version of Merian C Cooper’s King Kong which has been missing from the film company’s vaults since 1934.
Mr McMillan said: “I didn’t know what it was at first, I hoped it was hidden treasure but when I realised it wasn’t I was about to throw it in the skip.
“One of the cinema staff stopped me and took it away to find out what it was.
“When I heard it was an original version of King Kong, one favourite movies as a kid, I couldn’t believe it.”
The film is renowned for its pioneering special effects, stop motion models and early animation.
But the cinema projectionist got a shock after contacting the original distributor, when he discovered late fees for the film have mounted over the passing decades to more than £43,000 in today’s money.
Universal Pictures, which now owns RKO, has told cinema bosses it is willing to waive this fee to have the prestigious film reel back in its prized collection.
And as a special thank you, it will allow the Grosvenor Cinema to show the film a final time bringing 1930s cinematography back to the West End institution.
The iconic 1933 version of King Kong will be screened to celebrate the launch of The Grosvenor Café on Friday.
Conor Miskimmin, general manager of The Grosvenor Café said: “It’s incredible to think that the film has been hidden away for all that time. “The screening is a really fitting way to celebrate the launch of the Grosvenor Café.
“I can’t wait for the opening night and for the titles to roll. I’m just glad we don’t have to pay the fine!”
The Grosvenor Cinema dates back to the 1920s and underwent a multi-million pound refurbishment in 2002 with two cinemas at ground level and bar and restaurant above. Then, workmen removed a false plaster ceiling and uncovered an unused floor complete with curved art deco ceiling.
It was restored to feature an upstairs bar and restaurant where the King Kong rarity was discovered in the bar’s mezzanine level.
Film fans who want to get their hands on a ticket for the exclusive King Kong screening and drinks reception should contact the cinema box office on 0845 166 6002.