Elton John Calls Michael Jackson 'Mentally Ill' in New Memoir

Elton John says Michael Jackson came off as "genuinely mentally ill" late in life -- which left the Rocket Man himself believing the King of Pop had gone off the deep end.

The singer makes some pretty big claims about MJ in his new memoir, "Me," which is coming out Tuesday. Certain outlets have obtained an advanced copy of the book -- and one excerpt about Michael is making the rounds ... in which Elton discusses his mental state.

Per reports, Elton writes ... "I’d known Michael since he was 13 or 14 ... He was just the most adorable kid you could imagine. But at some point in the intervening years, he started sequestering himself away from the world, and away from reality the way Elvis Presley did."

He goes on to say, "God knows what was going on in his head, and God knows what prescription drugs he was being pumped full of, but every time I saw him in his later years I came away thinking the poor guy had totally lost his marbles."

Then comes the big allegation, with Elton writing ... "I don’t mean that in the light-hearted way. He was genuinely mentally ill, a disturbing person to be around."

Elton continues by describing a time he invited MJ to a party of his, claiming that the guy had wandered off at one point and was found playing with Elton's housekeeper's son. He says, "For whatever reason, he couldn’t seem to cope with adult company at all."

Michael died in 2009. He's been dragged back into the news with claims of sexual abuse by Wade Robson and James Safechuck, detailed in HBO's "Leaving Neverland."