Kevin Federline Accuses Britney Spears of Cheating on Him, With a Woman

Kevin Federline's new book "You Thought You Knew" details his first fight with his ex Britney Spears ... and it's a doozy -- KF says he caught her cheating with a female dancer!

In an excerpt from his new book, obtained by USA Today, Kevin says the alleged incident happened during a wild night on tour in Amsterdam, back at the hotel.

KF writes, "Britney and I went to our room, but shortly after, she said she’d be right back. At first, I didn’t think much of it. The hotel floor was ours; they had booked it out for the entire tour, so everyone’s rooms were nearby. But as time passed, I began to wonder what was taking so long."

KF continues ... "Eventually, I decided to check if everything was okay. 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. 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. It was one of those moments that hits you so fast, your brain can’t even begin to process it. You just stand there, stunned, not sure what’s real. I didn’t say a word. Just stood there, watching. For a second, nobody moved. Then they both looked up and saw me, and I turned and walked away. My heart was racing, but not with excitement. It was something colder, a sudden clarity. This wasn’t what I signed up for. So I got the hell out of there. As soon as I got back to our room, I grabbed my suitcase and started packing. At that point, I cared about nothing else – I just wanted out."

According to KF, when the "Toxic" singer returned to the room to find him packing up his things, she begged him to stay, promising it would never happen again.

"I could tell she meant it. But I was still furious," KF writes. "I wasn’t interested in a relationship that involved more than two people."

But, of course, Kevin did stay. He says that time in Europe laid the foundation of their rocky relationship ... "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."

"You Thought You Knew" is out October 21.