Kevin Federline says he remembers when he drew the line with ex Britney Spears .

In the former professional dancer’s new memoir You Thought You Knew , set to release Oct. 21, he alleged the first crack in his three-year marriage with the “Circus” singer came during a fight about her apparent drug use at the release party for his 2006 album Playing With Fire.

“The first thing I saw was Britney and her young starlet friend snorting a fat line of coke off the table,” Kevin—who says Britney was still breastfeeding their kids Sean Preston , now 20, and Jayden James , now 19, at the time—wrote in the book. “It was surreal. They didn’t even try to hide it.”

And while the 47-year-old says he wasn’t fazed by his then-wife’s drug use, he noted, “what was hitt

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