TOXIC BRITNEY FEARS K-FED'S POISON PEN!

Britney shares teen sons Sean and Jayden with her ex-husband

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POP tart Britney Spears is freaking out after learning baby daddy Kevin Federline is writing a tell-all book and insiders dish she's in a mad dash to get hers out before he steals her thunder!

"Britney's desperate to get her side of the story out there and panicking over what Kevin's going to reveal about her," dishes an insider.

"They're on very poor terms right now and she's also very nervous about giving him ammo to come after her with when she does release her own life story."