Runrig lead vocalist Bruce Guthro has died at the age of 62.
The Canadian singer-songwriter joined the Scottish rock band in 1998 and remained with the group until its final performances in 2018.
Runrig said Guthro died on Tuesday night after a years-long battle with cancer.
Friend and former band-mate, SNP MP Pete Wishart, said he had been an exceptional singer, musician and songwriter.
Runrig said: "It is with the heaviest of hearts and profound sadness that we inform you of the news that Bruce passed away last night, having finally lost a long battle with cancer that has stretched back many years.
"Everyone associated with Runrig is heart broken at the loss of a dear friend and such a special musical colleague."
Runrig said the band's thoughts and prayers were with his family.
Speaking to BBC Radio Scotland's Drivetime programme, Mr Wishart - who played keyboard for the band before becoming an MP - said everybody associated with Runrig was heartbroken.
He said: "We've lost a friend, but we will never get to hear this wonderful voice singing voice live again."
In a post on social media in July, Guthro said he had been battling health issues for a number of years and had been forced to cancel live performances.
Guthro was from Nova Scotia, Canada, and his award-winning music career of more than 40 years included work as a solo artist.
He joined Runrig after Donnie Munro left the band.
Mr Wishart said the group had almost given up looking for another lead singer when Guthro auditioned.
He said: "Bruce came in that day when Scotland were playing Brazil in the 1998 World Cup and we had half an eye on the football and half an eye on Bruce.
"When Bruce sang we turned around almost with our jaws hitting the ground with the quality of Bruce's voice."
Scottish fiddle player Duncan Chisholm said he had known Guthro as a friend for more than 25 years, and described him as incredibly gifted singer.
In a tribute, Gaelic singer Julie Fowlis said: "He was such a gifted singer and a powerful performer.
"It was one of the greatest privileges for me to sing with him onstage and backstage over the years."
Runrig's songs, many of them sung in Gaelic, span more than four decades and the band has a large and loyal following across Scotland and the rest of Europe.
The band enjoyed UK chart success in the 1990s and in 2018 sold-out their final performances, the two-day The Last Dance - Farewell Concert in Stirling.