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Kevin Federline claims Britney Spears was seriously "Toxic" ... literally, for her children.

K-Fed alleges the Grammy-winner drank alcohol while pregnant and on medication in an excerpt from his book, "You Thought You Knew," obtained by People . Getty

Kevin writes, "That mix was... dangerous. F----d up, honestly ... You’re not supposed to drink when you’re on meds like that. A couple of glasses of wine for her would hit like a whole bottle because of the medication.”

This comes despite Spears writing in her 2023 memoir, "The Woman In Me," she "never" had a drinking problem or "any interest in hard drugs."

K-Fed writes otherwise ... “I had seen it before, her drinking and breastfeeding, and it was really upsetting because of the danger to the kids ... She c

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