Sir Paul McCartney has revealed he had no idea he was recording with Nirvana when he jammed with the band's surviving members to create a Grammy-winning song.
The musician said he was unaware he was actually taking the place of late frontman Kurt Cobain in what turned out to be a brief reunion of the grunge kings and merely thought he was playing with Dave Grohl and two of his pals.
The track they produced, Cut Me Some Slack, went on to take the best rock song title at the Grammy Awards earlier this year and they have also performed it live.
But Sir Paul admitted that he did not recognise Dave's two pals who had joined them in the studio, and who turned out to be bass player Krist Novoselic and Nirvana's tour guitarist Pat Smear.
The group disbanded following the suicide of Cobain in 1994, with drummer Dave moving on to found and front Foo Fighters.
The former Beatle found himself playing with the Nirvana stars after agreeing to work on a track with Dave for his Sound City project in which he brought together guest musicians for an album and film using the mixing desk from a celebrated Seattle recording studio.
Sir Paul had said he would take part if he could bring along a cigar box guitar which he had recently received as a gift from Hollywood star Johnny Depp.
"He's a guitar freak and he'd given me this cigar box guitar, and I plugged it in and it was howling and it was great," Macca recalled as he spoke at an event in London organised by model Lily Cole.
"So I took my bass and this guitar and said 'I'd really like to play this guitar', and he said 'okay, great'.
"So, we got it out in the studio and he had two other mates with him, a really tall guy and a not-so-tall guy.
"They turned out to be two members of Nirvana, Krist and Pat, and I ended up playing with three Nirvana guys which I didn't know until about halfway through the session.
"They said 'this is great, we haven't played like this for years' and I said 'oh, you've played together?'
"It was just so exciting. I wasn't trying to be Kurt because I didn't even know they were Nirvana, so I was just jamming with some guys who I liked."
Sir Paul was speaking at an event for aspiring songwriters staged for Lily's Impossible community website in which he discussed how he composes songs, including his new single Hope For The Future which comes out on December 1. He initially wrote the track for the video game Destiny.