Tributes poured in for the German-born Spanish baritone on social media.
Il Divo singer Carlos Marin has died aged 53, the group has announced.
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.
Tributes to Marin poured in on social media, with TV presenter Lorraine Kelly tweeting: “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 humor we will miss you so sad.”