The singer died after suffering a suspected cardiac arrest, his brother said.

Jermaine Jackson said he believed the 50-year-old singer suffered the heart attack at his Holmby Hills home in Los Angeles yesterday and was taken to the UCLA medical centre where he was pronounced dead.

He said: "My brother, the legendary King of Pop, passed away on Thursday June 25 at 2.26pm.

"We believe he suffered a cardiac arrest at his home. However, the cause of his death is unknown until the results of the autopsy are known. The personal physician who was with him at the time attempted to resuscitated him."

The self-styled King of Pop was due to perform 50 farewell concerts in London this summer, bowing out on an illustrious music career spanning several decades while aiming to restore his fortunes.

A spokesman for the hospital said: "When he arrived at hospital at approximately 1.14pm a team of doctors, including emergency physicians and cardiologists, attempted to resuscitate him for a period of more than one hour. They were unsuccessful."

TV footage showed a rescue helicopter flying the star's body to a waiting ambulance.

Tributes from the world of music, film and celebrity have flooded in.

Singer and actress Cher told CNN presenter Larry King: "He was a great singer. You know, it's like God gives you certain gifts. And some people he gives different gifts and some people he gives more gifts.

"And this child was just an extraordinary child, touched by this ability to have people feel him and feel people. You know, he was a genius, like Ray Charles, like Stevie Wonder. They just have this gift."

Jackson's first wife, Lisa Marie Presley, told MTV: "This is such a massive loss on so many levels, words fail me."

Madonna said in a statement on the BBC website: "The world has lost one of its greats but his music will live on forever."

The head of the Sony Corporation, Sir Howard Stringer, said: "Michael Jackson was a brilliant troubadour for his generation, a genius whose music reflected the passion and creativity of an era."

And Justin Timberlake - who has been compared to a young Jackson - told MTV: "We have lost a genius and a true ambassador of not only pop music, but of all music."

A Los Angeles Police Department spokesman said the robbery and homicide team was investigating Jackson's death because of its "high profile". But he added that nothing should be read into his team's involvement.

A post-mortem examination is expected to take place later today, Los Angeles Coroner's Office spokesman Fred Corral said.

Jackson's close friend, Uri Geller, told the Press Association it may have been the stress of Jackson's London comeback that killed him.

Speaking from his home in Sonning, Berkshire, he said: "I guess the stress, the anticipation and the passion he was emitting from his heart, wanting to do this comeback so badly, maybe that got to him.

"That is all I can think, that maybe the stress killed him."

He described Jackson, who was best man when Geller renewed his wedding vows, as a "genius".

Fears were sparked for Jackson's health last month when four of his This Is It concert dates were pushed back but organisers insisted the dates were moved due to the sheer magnitude of the spectacle.

Randy Phillips, president and chief executive officer of AEG Live, said the date changes had "absolutely nothing to do with (Jackson's) health".

The Rev Al Sharpton, a leading US civil rights campaigner and friend of Jackson for 35 years, said the star was a "trailblazer" and an "historic figure".

"Michael Jackson made culture accept a person of colour way before Tiger Woods, way before Oprah Winfrey, way before Barack Obama," he said.

"Michael did with music what they did in sports, in politics, and in television.

"No controversy will erase the historic impact."

Matt Blank, from the Michael Jackson World Network fan club, told BBC News 24 he was "a bit dumbfounded and a bit shellshocked by it all".

He added: "I find it hard to believe this could be a reality.

"In the past, health issues of Michael Jackson have always been rumoured. I can only imagine this is down to the amount of stress he was under to come out and perform."

Jackson spent time living outside the United States since his 2005 acquittal on child molestation charges.

Jackson leaves three children, Michael Joseph Jackson Junior, Paris Michael Katherine Jackson and Prince "Blanket" Michael Jackson II.