The demo recording of It’s For You, written for Cilla Black by John Lennon and Sir Paul McCartney, was discovered by Black’s nephew Simon White. It had been given to his father by Black - his sister - when he was 18.
But while Mr White’s father assumed it was just a copy of his sister’s recording of the song, which shot to number seven in the charts, it was in fact the original demo sung by McCartney and delivered to Black at the London Palladium in 1964.
Black, who died just over a year ago at her home in Spain, wrote about the record in her book What’s It All About?.
The record is part of the 25th Beatles Memorabilia Auction, organised by The Beatles Shop, at the Unity Theatre in Liverpool on Saturday, and is expected to fetch between £15,000 and £20,000.
After first listening to the record, shop manager and Beatles expert Stephen Bailey said: “I was shaking with excitement and speechless.
“I realised that this was the long-lost Beatles demo disc from 1964 and I was probably one of the few people to have seen and heard it in over 50 years. What I had in my hand was probably the only copy in existence.”
Simon White donated the record with a letter, explaining how his father - an avid collector - had looked after it for years after Black gave it to him.
He wrote: “When we found out that Sir Paul had not retained his own copy of the recording, as a courtesy and gesture of respect we agreed it would be only right that, of all people, he be allowed to make a copy of the recording for himself for the sole use of adding it to his personal archive.
“Other than the single copy of the recording that he now has, we know of no other.”
Assistant store manager Anne-Marie Trace said: “It’s so hard to find something of The Beatles that hasn’t been released in some way or other. There has been interest from all over the world, and I expect it will end up going to a real fan.”
Mr Bailey added: “I urge all Beatles fans to search their bottom drawers and attics where they could have put Beatles memorabilia years ago and forgotten all about them - you just never know what you may find or what it is worth.”