Imagine . . . there’s no ceiling!

John Lennon’s long-lost Framus Hootenanny 12-string acoustic guitar goes on the auction block this month and the sky’s the limit on the price it could fetch — with estimates of more than $800,000.

Lennon used the 12-string in the 1960s, most famously while the Beatles were recording the albums “Help!” and “Rubber Soul,” as well as in the movie “Help” where he played “You’ve Got to Hide Your Love Away.”

George Harrison used it to play the rhythm guitar part on “Norwegian Wood” and it appears on another “Rubber Soul” track, “Girl.”

The guitar hadn’t been seen in more than half a century, but it was recently discovered in the attic of a home in the British countryside as the homeowners were moving.

The founders of Beverly Hills-based Julien’s Auctions, who is staging the May 29-30 sale at the Hard Rock Cafe in Times Square, traveled to Britain to verify the guitar and recovered the original case from the trash.

“Once in a while we get an incredible call,” said Darren Julien, co-founder and executive director of the auction house. “Finding this remarkable instrument is like finding a lost Rembrandt or Picasso, and it still looks and plays like a dream after having been preserved in an attic for more than 50 years.”

Julien’s determined Lennon gifted the guitar to fellow musician Gordon Waller, of the 1960s pop duo Peter and Gordon; Waller, in turn, gave the 12-string to its current owner, who believed it to be lost.

Noted Julien’s co-founder Martin Nolan, “The woodgrain of a guitar is like a fingerprint in that no two guitars are the same. 

Not only is the woodgrain a perfect match to the guitar that John and George are playing but so is the pickguard which can be exactly photo matched. 

Because the guitar has been undisturbed for approximately five decades, it is in the exact condition in terms of aesthetics that it was when John and George played it.”

The 12-string is “the most significant Beatles guitar to ever come up for auction,” Nolan said.

In addition to the Lennon guitar, instruments from Bob Dylan, Mark Knopfler, Robbie Robertson, Randy Bachman and Steve Jones of the Sex Pistols will be featured at the two-day “Music Icons” auction.