Michael Jackson had been using Propofol since 1999, according to his former personal assistant.

The 'Billie Jean' singer died in 2009 from acute intoxication of the anaesthetic - which is usually reserved for use in hospitals - and his one-time aide Frank Cascio claims he had been taking the drug for a decade.

Writing in his new memoir, 'My Friend Michael: An Ordinary Friendship with an Extraordinary Man', Frank claims he believes the pop superstar first started using painkiller Demerol after burning his head while filming a TV commercial for Pepsi in 1984 and that he first noticed him using it on his 'Dangerous World Tour' in 1993, which he believed was to treat a skin disease.

Frank - who was a friend of Michael from the age of five - claims his drug use had escalated so much that by 1999 he had started to take Propofol after injuring himself in a fall from a stage in Munich, Germany.

Frank writes he realised Michael's drug problems were escalating, and he sometimes paid his doctors in cash and put his own name on prescriptions to hide the singer's medical problems and the money he was spending.

He also writes in the book that he tried to get help from three of Michael's siblings before the 'Michael Jackson: 30th Anniversary Special' concerts which took place in 2001, but the singer "simply pushed them away".

He also claims the 'Beat It' hitmaker started the first of his anniversary shows an hour late because he had taken Demerol at his hotel.

Writing about the incident, he said: "My naive belief that Michael wouldn't let his medicine interfere with the show blew up in my face. I can't begin to describe my disappointment and panic at this moment."

Writing about his friend's death, he states he believes Michael - who frequently complained of sleeping problems - had "died in his endless quest to attain some inner peace".

Last Monday (07.11.11), Michael's personal physician Dr. Conrad Murray was found guilty of involuntary manslaughter for the part he played in the musician's death in June 2009

He faces up to four years in prison, and will be sentenced at the end of this month.