That’s toxic.

Kevin Federline’s marriage to Britney Spears was “never the same after” the singer “slapped” him in the face during an argument, he claims in his new bombshell memoir, “You Thought You Knew.”

The former backup dancer claims, in an excerpt obtained by Us Weekly , that the Princess of Pop went on a “rampage” after Federline’s pal dropped marijuana off at their home.

He alleges the Grammy winner got physical with him while holding one of their sons .

While the outlet did not reveal when the alleged altercation took place, the former couple welcomed Sean Preston and Jayden, respectively, in 2005 and 2006, calling it quits when their youngest was a month old.

Federline, whose divorce from Spears was finalized in 2007, writes that the slap “wouldn’t be the last time” th

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