Kevin Federline Accuses Ex Britney Spears of Cheating on Him With Female Backup Dancer: 'They Were Full-On Making Out'

Was Britney Spears the "Womanizer" herself?

The pop star's ex-husband Kevin Federline shockingly accused Spears of cheating on him with a female backup dancer while they were dating more in his upcoming memoir You Thought You Knew — which hits bookshelves on Tuesday, October 21.

An excerpt of the tell-all memoir exposed a night Spears and Federline went out in Amsterdam in 2004.

The DJ said they were joined by members of Spears' Onyx Hotel Tour crew — including Teresa Espinosa, who introduced Federline to Spears one month prior.

After a wild night in Europe, Federline and Spears eventually returned to their hotel room, though the failed rapper claimed the "Toxic" singer left and promised to be right back.

"As time passed, I began to wonder what was taking so long. I was thinking maybe she was catching up with someone or handling something tour-related," he wrote.

Becoming concerned, Federline left the hotel room to try and find his then-girlfriend.

"As I passed one of the rooms, I glanced inside — and froze. There she was," he alleged. "Theresa [sic] was sitting on the edge of the bed. Britney stood between her legs, hands on her face and they were f------- going at it. Full-on making out."

Spears and Espinosa supposedly spotted Federline before he stormed back to his hotel room to collect his belongings.

"I just wanted out," Federline penned, revealing he told Spears he was breaking up with her and leaving Amsterdam.

"She started apologizing, explaining, promising it would never happen again," he alleged. "She told me she didn’t realize how serious I was about us, that she was overwhelmed and it had been a mistake."

Federline informed Spears he "wasn’t interested in a relationship that involved more than two people" and decided to give her another chance.

He recalled, "Those weeks in Europe, navigating that emotional minefield together, laid the foundation of our relationship."

"They tested us, changed us and ultimately brought us closer together. It was during this time that I began to realize the depth of my feelings. And while the road ahead was anything but certain, I knew one thing for sure: I was falling in love with Britney," Federline declared.

Spear and Federline had only met about one month before he joined her in Amsterdam for the May 2004 stop of her tour.

After the alleged cheating incident, Federline and Spears stayed together and tied the knot by September of that same year.

They went on to welcome sons Sean Preston, 20, and Jayden James, 19, in September 2005 and September 2006, respectively.

Spears ultimately filed for divorce from Federline in November 2006, however, citing "irreconcilable differences."

Between their problematic relationship and the abusive conservatorship Spears was later put under for 13 years, the "Circus" hitmaker became estranged from her children but has since worked to mend their bond.