Simon Cowell has led the entertainment industry's tributes to singer Whitney Houston just hours after she was found dead in a Los Angeles hotel room.
The 48-year-old singer was in the city for the Grammy awards which are due to take place on Sunday night.
Music mogul Cowell, who was rumoured to be considering Houston as a judge on American X Factor, has spoken as other stars poured out their grief on Twitter.
"It's really, really horrible," he said in an interview with Heart FM. "She was a delight to work with."
Speaking of her infamous appearance on the British X Factor in 2009, he said: "She probably wasn't altogether there but it was still Whitney Houston and you knew you were in the presence of a legend. I'll never forget that day."
Many of the celebrities due to attend the Grammys have taken to Twitter to express their condolences to Houston's family and show their love for her.
American country music star Dolly Parton, who wrote I Will Always Love You, one of Houston's biggest hits, said: "Mine is only one of the millions of hearts broken over the death of Whitney Houston. I will always be grateful and in awe of the wonderful performance she did on my song, and I can truly say from the bottom of my heart, Whitney, I will always love you. You will be missed."
Katy Perry wrote: "So devastating. We will always love you Whitney, R.I.P." and Rihanna said: "No words! Just tears #DearWhitney."
Rapper P Diddy echoed the same sentiments saying: "Whitney Houston!!! OMG!! I really can't believe this.. This is one of the saddest days ever.."
Mariah Carey sang the track When You Believe with Houston in 1998 for the film The Prince of Egypt.
She tweeted: "Heartbroken and in tears over the shocking death of my friend, the incomparable Ms. Whitney Houston. My heartfelt condolences to Whitney's family and to all her millions of fans throughout the world. She will never be forgotten as one of the greatest voices to ever grace the earth."
And Justin Bieber wrote: "Just heard the news. so crazy. One of the GREATEST VOICES EVER just passed. RIP Whitney Houston. My prayers go out to her friends and family."
Soul legend Aretha Franklin, who was Houston's godmother, told reporters: "I just can't talk about it now. It's so stunning and unbelievable. I couldn't believe what I was reading coming across the TV screen. My heart goes out to Cissy, her daughter Bobbi Kris, her family and Bobby."
Mitch Winehouse, whose daughter Amy died last year after a long battle with drink and drugs, said: "We are in LA and just got the news about Whitney. We are absolutely devastated. Rest in peace Whitney."
And Michael Jackson's sister La Toya wrote: "I can't fathom the fact that #whitneyhouston has died! My heart goes out 2 her mother, daughter & her love ones. #Whitney U will be missed!"
"Six-time Grammy winner Whitney Houston was one of the world's greatest pop singers of all time who leaves behind a robust musical soundtrack spanning the past three decades," said Neil Portnow, president and CEO, The Recording Academy.
"Her powerful voice graced many memorable and award-winning songs. A light has been dimmed in our music community today, and we extend our deepest condolences to her family, friends, fans and all who have been touched by her beautiful voice."
Singer Lionel Richie told CNN: "She was just a gift in voice ... She was just so innocent in the business, and that voice was so pure and her life was so pure.
"That's what I'm holding on to. I think what we're going to do in the long run of her life story is focus on the voice, not the tragedy, but the voice."
England footballer Rio Ferdinand was also among those to pay his respects, tweeting: "RIP Whitney Houston...what a singer." Pop star Cheryl Cole also wrote: "So sad :-("
"I am absolutely heartbroken at the news of Whitney's passing," legendary music producer Quincy Jones said in a written statement.
"I always regretted not having had the opportunity to work with her. She was a true original and a talent beyond compare. I will miss her terribly."
Barbra Streisand wrote on Twitter: "She had everything, beauty, a magnificent voice. How sad her gifts could not bring her the same happiness they brought us."
Bee Gees singer Robin Gibb added his condolences, saying: "In my estimation we have lost one of the greatest voices in popular music today... but as we always learn too late... we never cherish it until it's gone. My thoughts are with her family."
Singer and actress Christina Aguilera wrote: "We have lost another legend. Love and prayers to Whitney's family. She will be missed."