Once believed long lost, a "legendary" demo of Sex Pistols' highly controversial song "Belsen Was a Gas" has now surfaced on the Internet.

Reportedly penned by Sex Pistols bassist Sid Vicious before he joined the influential punk rock band, the song is about a Nazi concentration camp in Germany.

As music magazine NME notes, the track became part of the Sex Pistols' live set from 1977. However, until recently, only live recordings and a soundtrack version featuring train robber Ronnie Biggs on vocals had existed.

The original demo version of the song features band frontman Johnny Rotten on vocals and Vicious on bass.

The demo was reportedly found when the group's back catalogue was transferred from Virgin Records to Universal Music Catalogue earlier this year, Australia's Herald Sun notes.

The song, which has sparked plenty of controversy throughout the years with its shocking lyrics and theme, is about the Bergen-Belsen concentration camp which was liberated by British troops in 1945. Contrary to the song's title, the concentration camp did not house any gas chambers. Those who perished there -- numbering at least 50,000 people -- died from starvation, disease and other factors.

Rotten later admitted the song had been in poor taste -- even for a band infamous for their unapologetic shock tactics and controversial music.

He said it was a "very nasty, silly little thing" which he claimed should have "ended up on the cutting room floor," Herald Sun notes.

Nonetheless, when Sex Pistols played a reunion tour in 2003 (sans Vicious, who died in 1979), the band -- in the wake of the beginning of the war in Iraq -- performed an adapted version of the song called "Baghdad Was A Blast."

According to NME, the demo track has been restored and will feature on the box set deluxe reissue of the band's seminal album "Never Mind The Bollocks, Here’s The Sex Pistols."

The box set is out on Sept. 24th in celebration of the iconic album's 35th anniversary.

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