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

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