Britney Spears is being accused of having cheated on Kevin Federline with a female dancer, according to the latter's upcoming memoir.

In "You Thought You Knew," out Tuesday, 47-year-old Federline -- who met and married the pop star, 43, in 2004 -- writes that he found Spears "going at it" with one of her female dancers during a European tour amid their "whirlwind" romance.

"As I passed one of the rooms, I glanced inside -- and froze. There she was. Theresa was sitting on the edge of the bed. Britney stood between her legs, hands on her face, and they were (expletive) going at it. Full-on making out," reads an excerpt published by USA Today.

The former backup dancer says the alleged encounter left him with "a sudden clarity."

"This wasn't what I signed up for. So I got the hell out of there," he writes. "At that point, I cared about nothing else -- I just wanted out."

Federline claims he tried to get "the first flight back to the States" when the "Oops! I Did It Again" singer returned to their room to find him packing and "started apologizing, explaining, promising it would never happen again."

"She told me she didn't realize how serious I was about us, that she was overwhelmed, and it had been a mistake," he writes. "That moment was a turning point for both of us. It was our first fight. ... I think it was the moment she realized I wasn't there for the fame or the perks. I was only there for her. ... The fact that I was ready to walk away ... proved it was real."

The exes, who divorced in 2007, share sons Sean Preston, 20, and Jayden James, 19, for whom Spears' child support ended in late 2024.

In an excerpt published by The New York Times, Federline writes that he worries "something bad is going to happen" to Spears, due to her mental health, and that their sons "will be left holding the pieces."

Federline's memoir comes nearly two years to the day after the Oct. 24, 2023 release of Spears' bombshell memoir, "The Woman in Me," which details their tumultuous marriage, as well as her relationship with Justin Timberlake that preceded it. As of August 2024, "Wicked" director and producer Jon M. Chu and Marc Platt are set to helm and produce a biopic based on the bestseller.

Kevin Federline alleges Britney Spears cheated on him with female dancer