Doctors have reportedly begged the "Thriller" singer to eat more or risk collapsing on stage during his 50-show residency at London's O2 arena in July.
Concert promoter AEG Live insist the 50-year-old star is "extremely fit" and looking forward to taking to the stage, but others claim he is dangerously underweight, and needs gain at least 20lbs to be considered healthy.
A source told Britain's The Sun newspaper: "There are concerns Michael is too frail. He has been ordered to eat more but he refuses to and eats like a bird."
Jackson is reportedly still fighting to overcome a skin infection he contracted in February and suffering from an old back injury.
The star has not performed live since his disastrous appearance at the 2006 World Music Awards, where he visibly struggled to sing two lines of "We Are The World," before staggering off stage.
He has spent the past two months in Los Angeles rehearsing for his "This Is It" series of shows, which are rumored to include live animals, a duet with his 12-year-old son Prince and performances from stars including Britney Spears and Leona Lewis.
