This isn't the first devastating allegation Demi has written about in her dramatic new memoir.
Laced with shocking confessions Inside Out details threesomes, infidelity and a miscarriage as catalysts for the couple's demise.
The couple, who have a 15-year-age gap, met in 2003 and married in 2005, before splitting in 2011 and finally divorcing in 2013.
In the book, Demi, 56, drops bombshell after bombshell about her ex-husband's bad behaviour, which she says began after she suffered a miscarriage when she was six months pregnant, and several attempts at IVF failed.
According to Demi when Ashton, now 41, was filming sexy movie scenes with Jennifer Jason Leigh, he told his wife that the actress had requested she didn't come to the set.
However, she later discovered the truth was quite the opposite.
“The bottom line was that she wasn’t worried about me being on set, he was,” she wrote. “He wasn’t honest.”
The mum-of-three daughters, who she shares with Bruce Willis, also confessed that she agreed to a threesome with Ashton for all the wrong reasons.
“I wanted to show him how great and fun I could be,” she wrote, but said their two trysts with a third partner was a “mistake.”