Il Divo singer Carlos Marin has died aged 53, the group has announced.
The German-born Spanish baritone's bandmates - American tenor David Miller, French singer Sebastien Izambard and Swiss tenor Urs Buhler - paid tribute to him on Sunday.
In a statement on Twitter, they said: "It is with heavy hearts that we are letting you know that our friend and partner, Carlos Marin, has passed away.
"He will be missed by his friends, family and fans.
"There will never be another voice or spirit like Carlos.
"For 17 years the four of us have been on this incredible journey of Il Divo together, and we will miss our dear friend.
"We hope and pray that his beautiful soul will rest in peace. With Love - David, Sebastien and Urs."
On December 16, the group had posted that Marin was "in the hospital" and that they were "hoping and praying for a speedy recovery".
The cause of death was not made clear, though reports in Spanish media suggested Marin was intubated after being admitted to an intensive care unit in Manchester.
Il Divo had postponed the rest of their December UK tour dates by a year "due to illness".
Marin had been on stage performing earlier this month.
The classical crossover group, comprised of a global melting pot of singing stars, was formed by Simon Cowell in 2003 under his SyCo record label, though they later parted ways with the music mogul.
In his personal life, Marin was married to French-born singer Geraldine Larrosa - better known by her stage name Innocence - until 2009.
Cowell led a flood of social media tributes to Marin, writing on Twitter he was "devastated" by the news.
"I am finding this so difficult to put into words how I feel right now," Cowell said.
"I am devastated Carlos Marin has passed away. He loved life. He loved performing and always had so much appreciation towards the fans who supported the group from day 1. Rest in peace Carlos. I will miss you."
Australian singer-songwriter and former frontman of pop duo Savage Garden, Darren Hayes, said on Twitter: "To the family, friends and fans of Carlos Marin I send my deepest condolences ... I'm so sorry for your loss."
Television presenter Lorraine Kelly tweeted: "This is so sad. I had the joy of interviewing Carlos and Il Divo many times over the years.
"He was such a class act. Thoughts with everyone who loved him".
Bruno Tonioli, a judge on the BBC's Strictly Come Dancing, wrote: "Devastated @ildivoofficial @carlosmarin_ passed away.
"We had the best time putting together the first @ildivoofficial performance 17 yeas ago great voice great man a true passionate Spirit with wicked sense of humour we will miss you so sad."
Australian Associated Press
