Dan McCafferty has died age 76, his former Nazareth bandmate announced.
The Scottish singer's death was confirmed by his friend and Nazareth bassist Pete Agnew.
He wrote on the band's Facebook page on Tuesday: "Dan died at 12:40 today.
"This is the saddest announcement I ever had to make. Maryann and the family have lost a wonderful loving husband and father, I have lost my best friend, and the world has lost one of the greatest singers who ever lived.
"Too upset to say anything more at this time."
Tributes to the singer began pouring in in the comments as one fan wrote: "So sorry to hear this, what a fantastic man all round, condolences to all Dan's family, friends & all the lads in the Band.. xx".
"I'm absolutely gutted to hear this news. Rest in peace Dan, my thoughts are with his family, friends and of course the band," another added.
And a third wrote: "Heartfelt Condolences to Dan’s family, friends & his Nazareth family. Dan will be greatly missed by all. X".
Dan co-founded Nazareth with bassist Pete, guitarist Manny Charlton and drummer Darrell Sweet in 1968.
The band was influenced by The Beatles, The Rolling Stones and Jimi Hendrix.
The band continued to perform and tour until Dan's retirement in 2013, meaning they performed for 45 years.
Dan and the band's best known album, Hair of the Dog, was released in 1975.
The album's title track, also a hit single, was later covered by Guns N' Roses in 1993.
Dan was forced to retire in 2013 due to health reasons, after he collapsed on stage – weeks leading up to the announcement.
The singer suffered with Chronic obstructive pulmonary disease (COPD).
An announcement on the band's website read: "We sadly have to tell you all that Dan has been forced to retire due to ill health.
"It has gotten worse lately, and after trying to play a festival in Switzerland last weekend, Dan found it impossible to continue after performing only three songs. In the dressing room afterwards, he told the band that he couldn’t go on any longer."
The band continued to tour and record as bassist Peter stepped into the leadership role, with Dan's blessing.
Speaking of his retirement, Dan previously said: "If you can't do the job, you shouldn't be there. Nazareth's too big for that."
Despite retiring from the band, Dan played with the idea of recording music again.
He told Classic Rock after his retirement: "I could always make another record.
"But getting up to do an hour and three-quarters, and get people to pay money to come and see me – I can’t do that."
