The supergroup have been working on updating 'Holy Man', which was co-written by late Beach Boys member Dennis Wilson and his lead singer brother Brian, 75.
Dennis' co-writer Stephen Kalinich is the only person who was able to hear the song in its original form and he claims he hasn't had access to it for more than three decades.
The 'Little Bird' drummer tragically drowned at Marina Del Rey in 1983, and the track has been stored away in a vault ever since.
A source said: ''The song is in a vault and nobody can do anything about it.
''It's such a shame as there will clearly be a huge demand for it.''
Dennis's co-writer Stephen says it is ''such a shame'' that no one will be able to get their ears on the ''six-minute masterpiece'' because of the law.
He told The Sun newspaper: ''It's over six minutes long and it's a masterpiece, but we've never been able to release it because we can't get the legalities worked out. Such a shame.
''People hear it and, honest to God, it brings tears to their eyes.''
Foo Fighters' drummer Taylor Hawkins recorded vocals on their version and Stephen says he sounded just like Dennis with ''the same gravelly, whisky voice''.
He added: ''Taylor sent the track to Brian May and he and Roger Taylor fell in love with it. They spent weeks fooling around with it, putting on vocals and some incredible percussive things.''