Boyzone singer Stephen Gately, 33, died on Saturday while on holiday in Majorca off the coast of Spain, the Irish band with a string of hits in the 1990s said on its website.
"Stephen tragically died yesterday whilst on holiday with his partner Andrew in Majorca," the site said on Sunday.
The Boyzone members, including lead singer Ronan Keating, were heading to Majorca following news of the death.
The circumstances surrounding Gately's death remained unclear but (Britain's) News of the World newspaper reported he died after a night out.
Boyzone manager Louis Walsh told the newspaper: "We're all absolutely devastated."
He said: "I'm in complete shock. I was only with him on Monday at an awards ceremony. We don't know much about what's happened yet... He was a great man."
Speaking to the newspaper late Saturday, band member Shane Lynch said: "Me and the boys are flying out in the morning.
"We just need to get over to where he's passed and work out what we need to do."
Gately, who married his partner Andrew Cowles in a civil union in 2006, sent ripples through the pop music world 10 years ago when he announced that he was gay.
He joined Boyzone in 1993 after answering an advert in Dublin to audition for Ireland's first boyband.
The band went on to enjoy huge success with six number one singles in Britain, but split up in 2000.
They reunited seven years later, but their recent 19-date Better tour failed to fill stadiums, despite offers of free tickets.
Gately also starred in West End musicals in London, including "Joseph and The Amazing Technicolor Dreamcoat".
Boyzone lead singer Keating was "completely distraught", the News of the World said, and former Irish prime minister Bertie Ahern paid tribute to Gately, whom he knew, on the BBC's Andrew Marr show.
He said: "The Gatelys are in my own constituency. I know the family, I know Stephen.
"He (Gately) was with us the other night with his boyfriend Andy, he came to my book launch."
Ahern, whose daughter Georgina is married to Nicky Byrne of boyband Westlife, continued: "It's just such a tragedy.
"Boyzone and Stephen, they've all been part of Irish life and far wider than that, the last 15 years, and so successful, so it's a huge, huge tragedy. It's so sad."
Gately last posted a message on his Twitter page on October 6, writing: "Still busy, lots going on. Focusing on finishing my book next so may be quiet here."
British actor and comedian Stephen Fry said on Twitter that he was shocked at the news.
"Just heard the very sad news about dear Stephen Gately. What a dreadful shock. He was loveable and sweet natured and will be hugely missed."
