Kasabian star Tom Meighan is stepping down from the band after 23 years.
The singer behind hits like Fire, LSF and Empire is leaving "by mutual consent", the band said in a statement.
"Tom has struggled with personal issues that have affected his behaviour for quite some time," they said.
The 39-year-old "now wants to concentrate all his energies on getting his life back on track", the Leicester band added, before concluding: "We will not be commenting further."
Meighan was a founding member of the band in 1997 alongside guitarist Serge Pizzorno, naming themselves after Linda Kasabian, a member of the Charles Manson cult.
They went on to become one of England's biggest rock bands, with five of their six albums going to number one. They headlined the Glastonbury Festival in 2014 and won best British band at the 2010 Brit Awards.
Last month, the group were due to play Solstice II - a huge homecoming gig at Leicester's Victoria Park - which was postponed due to the coronavirus pandemic.
Meighan opened up about his mental health in 2017, saying he had gone through a rough patch after separating from his partner Kim James, with whom he has a daughter, and the death of a close friend.
"Basically my life changed. I'm by myself. Because I lost myself," he told Q Magazine.
"I had to sort my head out. My attitude. Stuff I was doing. People I was associating with. Not bad people. I was the one that was bad."
He added: "I was making myself ill, I ain't gonna lie, my mind was jolted. I wasn't taking responsibility and it affected everyone around me, horrendous."
Making Kasabian's next album For Crying Out Loud "probably saved my life", Meighan later admitted to the NME.
"It was just an escape for Tom to go, 'This is my band and these tunes are exciting,'" said his bandmate Pizzorno. "Being in the studio made him realise the good things that were going on."
Just a couple of weeks ago, Meighan said work had commenced on Kasabian's seventh album.
"We need a seventh baby," he told Sky News. "We're going to try and make a new record as soon as we can but we can't really do anything while we're restricted."
That interview suggests Meighan's departure from the band has been relatively abrupt. There is no indication on whether they will continue as a three-piece or hire a new frontman.