A rare vinyl pressing of God Save the Queen by the Sex Pistols is expected to fetch up to £15,000 at auction.
The single was originally recorded for A&M Records in 1977, but the label dropped the band before it was issued and most of the copies were destroyed.
Martin Hughes, from Wessex Auction Rooms, near Chippenham, said: "It's a holy grail item... and as far as I can tell it's never been played."
In 2011 it was named the most valuable vinyl disc of all time.
While other records have since sold for considerably higher prices, the Pistols disc remains among the most legendary artefacts of its kind.
It was pressed for A&M in March 1977 but withdrawn within days after the band were sacked because of their notorious behaviour.
Shortly afterwards it was re-released by Richard Branson's Virgin Records to coincide with the Queen's Silver Jubilee.
It officially reached number two in the UK charts, despite being banned by the BBC, and is believed by some seasoned rock observers to have been the "real" number one, outselling its nearest rival by two to one.
Mr Hughes said it was thought only nine copies of the single were kept "in a vault" and handed out to a few long-standing employees when the London office closed in 1998.
He said he was a "bit sceptical" when the seller emailed him saying he had a copy, as it was "one of the rarest and most sought-after records in the world".
"He sent me pictures and it was undeniably the right thing," he said.
"And then I saw the all important 'golden handshake' letter and it just blew my mind."
Mr Hughes said the lot had attracted considerable interest from the US, Russia and Japan as well as the UK.
"The Sex Pistols are the most iconic punk band... It will be the most exciting thing I've sold ever," he said.