Britney Spears Accuses Justin Timberlake of Cheating in 'Woman in Me' Memoir

Britney Spears took most of the blame for years after she and Justin Timberlake split due to speculation she cheated -- but she's accusing him of doing the same with another celebrity -- one of many revelations in her new memoir.

Britney's "Woman in Me" doesn't drop until next week, but the cheating claim is just another chapter when it comes to her relationship with the *NSYNC star.

Remember, we were first to report Britney also says she and Justin got pregnant when they were 19, and she claims he pushed for the abortion she eventually had.

In the book, we're told Britney doesn't name the person with whom Justin allegedly stepped out, saying the woman now has a family Brit doesn't want to embarrass ... which means fans will, inevitably, start trying to connect dots on their own.

Of course, Justin dropped his massive hit "Cry Me a River" in 2002 -- admitting he wrote it after a nasty argument with Spears -- with many believing she had cheated on him.

Britney never denied the claims, and released her song "Everytime" the following year -- a song her writing partner says was a direct response to "Cry Me a River." Britney's lyrics in the song include, "I may have made it rain. Please, forgive me. My weakness caused you pain. And this song's my sorry."

However, since we broke the story of the abortion, some fans have speculated "Everytime" is actually about the pregnancy and abortion ... especially because the music video features a woman giving birth at the end.

Britney and Justin first met while working together on "The Mickey Mouse Club" in 1992 and then dated from 1999 to 2002.