The Beatles recorded the track in January 1967 for a music festival
Sir Paul McCartney has confirmed a 14-minute long Beatles track many thought was a myth does exist - and says he wants the public to hear it.
He told BBC Radio 4's Front Row the track - called Carnival of Light - was not released because the other Beatles thought it was too "adventurous".
The improvised track was recorded in 1967 for an electronic music festival.
Sir Paul said Ringo Starr and John Lennon's and George Harrison's estates would have to agree to a release.
He said he had been asked to create the piece for an electronic music festival, and asked the other band members to be "indulgent" for 10 minutes at London's Abbey Road studios before giving them vague directions.
Sir Paul explained: "I said all I want you to do is just wander around all the stuff, bang it, shout, play it, it doesn't need to make any sense. Hit a drum then wander on to the piano, hit a few notes, just wander around.
"So that's what we did and then put a bit of an echo on it. It's very free."
The track was played just once, at the festival, and is said to include distorted guitar, organ sounds, gargling and band members shouting phrases such as "Barcelona!" and "Are you all right?"
Sir Paul said he was fond of the track, which was inspired by experimental composers John Cage and Karlheinz Stockhausen: "I like it because it's The Beatles free, going off piste."
He said he still had a master tape of the piece, adding: "The time has come for it to get its moment."
Sir Paul had wanted to include the track on the Beatles' Anthology compilations in the mid-1990s, but the rest of the band vetoed the idea.
He said: "I said it would be great to put this on because it would show we were working with really avant-garde stuff."
The full interview will be broadcast on BBC Radio 4's Front Row on Thursday 20 November at 1915 GMT.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.