Watch: Stars pay tribute to Darius Campbell Danesh who has died aged 41
The singer and theatre star was found dead in his US apartment in Minnesota on 11 August, his family announced. Cause of death is unknown but there were said to be no suspicious circumstances.
Cowell, who was a judge on Pop Idol 2002 in which Danesh finished third, said in a statement: “I first saw Darius on television over 20 years ago and I got to know him really well.
“He was charismatic, funny and just a great person to be with.
Read more: Pop Idol star Darius Danesh has died aged 41
“His passing, for someone so young, is an absolute tragedy and my heart goes out to his family and friends."
Judge Nicki Chapman tweeted: "There will only ever be one Darius Campbell Danesh. A true gent with a kind soul & mischievous tinkle in his eye. From that eventful first audition on Popstars to him kindly inviting me to see him perform on the West End stage Thoughts are with his family at this sad time."
And Pop Idol judge and music producer Pete Waterman told TalkTV: "I had tea with him two days before lockdown on the Wednesday before and we went to tea.
"I met him… he was full of ideas. He wanted to get back into the music industry, wanted me to go over and look after him and mentor him through his meetings. He was doing a film in Hollywood.
"But you know what? Truth is with Darius, he was so brilliant at everything he did, he could never concentrate on one thing.
"So, I knew I've got a friend but I knew there was gonna be no business partnership. I just knew that was not part of what the deal was. He just could not concentrate."
Myleene Klass - who appeared with Danesh when he auditioned for ITV's original singing talent show Popstars in 2001 - shared a picture of her and Danesh with other pop hopefuls during a group meal.
She tweeted: "Back at the very start. RIP Darius. x"
Back at the very start. RIP Darius. x pic.twitter.com/eEoLi8mL69
— Myleene Klass (@KlassMyleene) August 16, 2022
Klass, 44, added a clip of Danesh singing Dream the Impossible Dream on Popstar to Operastar in 2010, saying: "Everyone should watch this performance. Darius, you were so talented, so kind. A true family man. My thoughts are with them now."
Gareth Gates, who worked with Danesh on Pop Idol, said on Instagram: "I can’t quite believe the news of my dear friend Darius passing. I’m heartbroken.
"Darius took me under his wing during our time in Pop Idol, like a big brother; always looking out for me and for that I thank you my friend.
"I will miss our chats. He was one of the most beautiful, intelligent and gentle souls I was lucky enough to know, a soul that lit up every room.
"Good bye my friend. Forever in my heart."
Danesh was said to be weeks away from appearing on a Pop Idol reunion show. And a last picture of him seen in public shows him walking a dog in Malibu with fellow Scottish star, Hollywood actor Gerard Butler.
He was best known for coming third in the first series of Pop Idol in 2002, won by Will Young.
After the show ended, he had a Number One hit with the single Colourblind in 2002.
He won the first series of ITV’s Popstar To Operastar contest, launching his stage career.
A few months after his win on the ITV show, which featured Welsh singer Katherine Jenkins as one of the judges, he landed his first major opera role as he was cast in a new production of Carmen at London’s O2 Arena.
He also starred in productions of Chicago, Guys and Dolls, and Gone With the Wind, crediting his early success on Cowell's talent show with helping him build a theatre career.
Danesh lived in the US and found love with Canadian actress Natasha Henstridge, 48, whose films include sci-fi hit Species.
The couple married in 2011 at the San Ysidro Ranch in Santa Barbara, California, but were divorced a few years later.
Danesh was born in Glasgow to a Scottish mother and Iranian father and was the eldest of three boys.
Although he found fame as Darius Danesh, he later changed his name to incorporate his mother Avril’s maiden name, Campbell, after an emotional visit to his grandfather in a Paisley nursing home made him reflect on his Scottish heritage.
He became a supporter of cancer charities including Cancer Research UK after both his parents were diagnosed with cancer and recovered.
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He also made a foray into Hollywood as a co-executive producer on the horror film Imperium starring Daniel Radcliffe in 2016.