Carnival Of Light was recorded for two events at the Roundhouse venue in London, which took place on 28 January and 4 February 1967. The events were known variously as The Million Volt Light and Sound Rave and the Carnival Of Light Rave, and the recording was played several times.
McCartney was approached by one of the promoters, David Vaughan, who asked him to make a recording for the event. The piece, made under the name 'Untitled', was taped in a single take on 5 January 1967 at a recording session for Penny Lane.
The existence of Carnival Of Light first came to light in Mark Lewisohn's 1988 book The Complete Beatles Recording Sessions. Since then it has become one of the most sought-after unreleased Beatles recordings.
We were listening to everything we'd every recorded [for Anthology]. I said it would be great to put this on because it would show we were working with really avant-garde stuff... But it was vetoed. The guys didn't like the idea, like 'this is rubbish'.
We were set up in the studio and would just go in every day and record. I said to the guys, this is a bit indulgent but would you mind giving me 10 minutes? I've been asked to do this thing. All I want you to do is just wander round all of the stuff and bang it, shout, play it. It doesn't need to make any sense. Hit a drum, wander 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.
I like it because it's The Beatles free, going off piste. The time has come for it to get its moment.
The full interview is to be broadcast on BBC Radio 4's Front Row on Thursday 20 November at 1915 GMT.