‘Furious’ Kevin Federline claims Britney Spears cheated on him with a female backup dancer

Kevin Federline is hurling more accusations at Britney Spears.

In USA Today’s “You Thought You Knew,” Federline, 47, claims that Spears, 43, cheated on him with a female backup dancer.

The incident allegedly happened in Amsterdam in 2004 when the then-new couple was joined for a night out by Spears’ Onyx Hotel Tour crew, including Teresa Espinosa, who had introduced Spears to Federline one month earlier.

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Federline writes that after he and Spears returned to their hotel room, the “Toxic” singer left and told him she would be right back. Federline, who initially assumed “she was catching up with someone or handling something tour-related,” eventually went to check on Spears.

“I stepped into the hallway, our door still cracked, like most on the floor, and headed toward the elevators. As I passed one of the rooms, I glanced inside — and froze,” he writes. “There she was. Theresa [sic] was sitting on the edge of the bed. Britney stood between her legs, hands on her face, and they were f–king going at it. Full-on making out.”

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“It was one of those moments that hits you so fast, your brain can’t even begin to process it,” Federline continues. “You just stand there, stunned, not sure what’s real.”

Federline said Spears and Espinosa saw him, so he ran back to his room and started packing his stuff.

“At that point, I cared about nothing else — I just wanted out,” he says, adding that when Spears came back into the room, he let his then-girlfriend know that he was leaving.

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“She started apologizing, explaining, promising it would never happen again,” the DJ recalls. “She told me she didn’t realize how serious I was about us, that she was overwhelmed, and it had been a mistake. For a moment, I saw her vulnerability. She was desperate for me to stay. Begging me. I could tell she meant it. But I was still furious. Still hurt. I told her that I wasn’t there for the ‘extra f–k,’ as I called it.”

Federline also said that he “wasn’t interested in a relationship that involved more than two people,” and while he “wasn’t judging” Spears, he didn’t want the incident to “set the tone” for their relationship.

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“I didn’t think about it then, but that moment was a turning point for both of us,” Federline writes. “It was our first fight, or whatever you want to call it. I think it was the moment she realized I wasn’t there for the fame or the perks. I was only there for her.”

“Still, there were lingering doubts,” Federline adds of the relationship. “I couldn’t shake the nagging voice in my head that wondered if I had made the right choice.”

The Post has reached out to reps for Spears and Espinosa for comment.

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Spears and Federline met in April 2004 and got married that September. They had two kids, sons Sean Preston and Jayden James, before splitting up in November 2006. They finalized their divorce the following year.

In “You Thought You Knew,” out Oct. 21, Federline reportedly sounds the alarm about Spears’ current “situation” and cautions that she is “racing toward something irreversible.”

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“Something bad is going to happen if things don’t change, and my biggest fear is that our sons will be left holding the pieces,” Federline writes in his memoir, per an excerpt obtained by the New York Times.

Federline also claims that Spears would wake up their sons in the middle of the night with a knife in her hand.

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Spears’ rep fired back at the singer’s ex-husband’s claims, accusing Federline of “profiting off” the pop star.

“All she cares about are her kids, Sean Preston and Jayden James, and their well-being during this sensationalism,” the rep said in a statement.