Stuart "Woody" Wood of the Bay City Rollers said the group's T In The Park gig will be his last with the band.
The guitarist, who enjoyed massive fame with the Rollers in the 1970s before a successful comeback last year, announced the news hours before the Rollers took to the stage at Strathallan Castle estate in Perthshire.
There have been rumours of a dispute between members of the veteran Scottish group, whose hits include Shang-A-Lang and Bye Bye Baby.
Woody wrote on Facebook that "disappointed is an understatement, but outwith my control".
The musician was quickly inundated with messages of support and sorrow from fans across the world.
Band members Les McKeown, Alan Longmuir and Woody revealed they were getting back together last autumn and tickets for a series of reunion gigs in Glasgow in December sold out in minutes.
Formed at the end of the 1960s, the group had a huge following at home and abroad with their distinctive tartan outfits and upbeat pop tunes.
They split in 1978, with bitter battles over money between band members and managers dragging on for decades, but set in motion a new wave of "Rollermania" with their recent return to the concert circuit.
Speaking before Woody's announcement, Les said he was looking forward to bringing something different to T In The Park with the "old guys".
The singer said: "A lot of people, especially at festivals, they love the diversity of music.
"Some maybe come for one band but they'll go and see a whole different eclectic mix of things.
"They'll be interested to see a band like us I think. A heritage band. The old guys."
Speaking about the opportunity to perform at Scotland's biggest music event, Les said: "I never thought we would be invited, but things have changed for the Bay City Rollers over the last six months.
"We've packed out audiences, we've done lots of great shows - especially in Scotland at the Barrowlands and at the Usher Hall.
"We've had a lot of great things happen to us and we're looking forward to more success. This is just another part of it."
He added: "All over the world there's different things happening. In Japan they are crazy about us still and scream even though they're 40 and 45.
"If I had my choice I think I would bring everybody from the other countries to Scotland and just perform for them."
Les will release a solo album on August 22 called The Lost Songs.