The 12-string guitar had been lying unused in the loft at a secret address in the English countryside for more than 50 years.

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Its owner had assumed he’d lost the guitar – until his son found it while they were moving house.

A check by experts revealed the guitar could fetch more than half a million at auction – because it was once played by John Lennon.

The music legend and his fellow Beatle George Harrison both played the guitar in sessions for the band’s albums Help! and Rubber Soul.

Lennon also used the instrument to play the rhythm guitar part on much-loved Beatles track Norwegian Wood.

Auctioneer Darren Julien said: “Finding this remarkable instrument is like finding a lost Rembrandt or Picasso.

“It still looks and plays like a dream after having been preserved in an attic for more than 50 years.”

Julien added: “To awaken this sleeping beauty is a sacred honour.”

Lennon had given the guitar to 1960s pop star Gordon Waller, who gifted it to its current owner.

Every guitar has a unique “woodgrain” like a person’s fingerprint – and the attic instrument’s woodgrain matched Lennon’s old guitar.

Julien told the Guardian: “Not only is the woodgrain a perfect match to the guitar that John and George are playing.

“So is the pickguard, which can be exactly photo-matched. The guitar has been undisturbed for approximately five decades.

“It is in the exact condition that it was when John and George played it.”

The guitar will go up for auction on May 29 and 30 at the Hard Rock Cafe in New York.

Auctioneer Julien said he expects the guitar to smash its guide price of $800,000 (£637,988).