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Britney Spears ' ex-husband Kevin Federline is raising concerns about her in his new memoir ... he says she's not OK since her conservatorship ended, and writes, "The truth is, this situation with Britney feels like it's racing toward something irreversible."

K-Fed's got a new memoir, "You Thought You Knew," dropping next Tuesday, and he sat down for an interview with The New York Times and gave the newspaper an advance copy ... in the book he writes, "Something bad is going to happen if things don't change, and my biggest fear is that our sons will be left holding the pieces." Listenin

Britney's ex says it's "become impossible to pretend everything's OK" with Britney and adds ... "From where I sit, the clock is ticking, and we're getting close to the 11th hour."

Kev

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