He collapsed and stopped breathing after an injection of a powerful painkiller named Demerol.
Jacko, 50, was said to be addicted to the drug – similar to morphine – and it is feared he took an overdose.
Paramedics who raced to his Los Angeles home after an emergency call found he had no pulse. And frantic attempts to revive him failed.
The Thriller star, who had been fighting skin cancer, was due to start a series of comeback concerts in London next month.
Instead, millions of shocked fans around the globe are today mourning a legend.