In her upcoming memoir, Britney Spears reportedly claims Justin Timberlake cheated on her with another celebrity while they were dating.

TMZ reports that Spears makes the bombshell claim in her new memoir, The Woman in Me.

According to the tabloid, Spears does not name the other celebrity in her book, as she "doesn't want to embarrass" them because they have a family now.

Spears and Timberlake dated from 1999 to 2002.

After their breakup, Timberlake released his hit song "Cry Me a River," which is believed to be about his relationship with Spears.

The music video for the single, which is about a cheating ex-girlfriend, features a Spears look-alike. At the time, Spears was infamously skewered by the media after many assumed she had cheated on Timberlake.

READ MORE: Britney Spears Eyeing Comeback Album Deal Amid Divorce: REPORT

The report that Spears claims Timberlake allegedly cheated on her with another celebrity comes just a day after the revelation Spears had an abortion in the early 2000s when she got pregnant by Timberlake.

"But Justin definitely wasn’t happy about the pregnancy. He said we weren’t ready to have a baby in our lives, that we were way too young," Spears writes in her memoir, according to People. 

"I don’t know if that was the right decision. If it had been left up to me alone, I never would have done it. And yet Justin was so sure that he didn’t want to be a father," she continues, calling it "one of the most agonizing things I have ever experienced in my life."

After the excerpt was published by People, some fans theorized that Spears wrote her song "Everytime" not just about her split from Timberlake, but also about the abortion she was convinced to have.

Spears herself has not confirmed whether or not the fan theory is true.

The Woman in Me hits bookshelves Oct. 24.

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Gallery Credit: Seth Berkman