The singer suffered a cardiac arrest at his home in Los Angeles last night, just weeks before he was due to begin a 50-date "comeback" residency in London.
He was taken to UCLA Medical Centre in the city in a coma but doctors were unable to revive him. His unexpected death has devastated fans and sparked a flood of tributes from the biggest names in the music industry.
His brother Jermaine Jackson formally confirmed the news outside the hospital: "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."
Jackson is widely credited with revolutionising pop music with albums including Thriller, Bad and Dangerous but his career was tainted with allegations of child abuse.
While lauded by fans and critics for his infectious pop songs, pioneering dance moves and innovative videos, his increasingly eccentric behaviour earned him the nickname "Wacko Jacko".
The Los Angeles coroner, Fred Corral, said that a post mortem examination would take place today to establish a cause of death. His body has already been flown by helicopter to the coroner’s office.
Paramedics who responded to an emergency call to the singer’s home in the Bel Air area of Los Angeles said that he had stopped breathing by the time they arrived. Police robbery-homicide detectives have been ordered to investigate, which is common in high-profile cases.
Quincy Jones, the music producer who worked with Jackson on Thriller, said: "I am absolutely devastated at this tragic and unexpected news."
Paying tribute to the singer’s "talent, grace, professionalism and dedication", he added: "I’ve lost my little brother today, and part of my soul has gone with him."
Madonna said: "I can’t stop crying over the sad news. I’ve always admired Michael Jackson – the world has lost one of its greats but his music will live on forever.
"My heart goes out to his three children and other members of his family. God bless."
Britney Spears said: "I was so excited to see his show in London. We were going to be on tour in Europe at the same time and I was going to fly in to see him. He has been an inspiration throughout my entire life and I’m devastated he’s gone!"
The Rev Al Sharpton, the civil rights campaigner, described Jackson as a "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.
Jackson’s mother Katherine and sister La Toya had rushed to the singer’s bedside at the hospital, where hundreds of tearful fans have gathered.
There was speculation that the pressure of his forthcoming London dates may have been too much for Jackson.
AEG Live, which organised the concerts at the O2 Arena in Greenwich, said Jackson had passed a lengthy physical exam in early 2009, but he last toured 12 years ago and looked markedly frail at the press conference to promote the shows.
Fears for his health emerged after he postponed the earliest dates, and his aides were forced to deny that he was fighting skin cancer.
Max Clifford, the publicist and friend of Jackson, said: "You wonder if the strain of getting fit for this major tour proved too much. In recent pictures he looked anything but healthy. He was always someone who seemed to find it difficult to cope with fame."
Another friend of Jackson, the illusionist Uri Geller, said there had been no indication that the singer was in a frail condition.
"I really have no words. He was a young and terribly fit man and he was getting ready for performances in England. He was just fine, the last I heard."
Jackson lived as a virtual recluse following his 2005 acquittal on charges including child molestation and kidnap.
He repeatedly denied undergoing cosmetic surgery, despite very visible changes to his face and skin tone, and was criticised for forcing his children to wear veils whenever they were in public.
Born in 1958, Jackson made his musical debut with four of his older brothers in the Jackson Five before embarking on a solo career.
His 1982 album Thriller – which included the hits Beat It, Billie Jean and Thriller – is still the best-selling album of all time, with more than 26? million copies sold. His lifetime sales tally is estimated at 750? million.
In 1994, he married Lisa Marie Presley, the 26-year-old daughter of Elvis Presley. The couple separated two years later and Jackson later married Debbie Rowe, a 37-year-old nurse he met while undergoing plastic surgery in 1997. They had two children, Prince Michael and Paris Michael Katherine, before divorcing in 1999.
Jackson had custody of the two children and of a third, Prince Michael II, whose mother’s name has never been made public.