United Kingdom, Oct. 14 -- Britney Spears' ex-husband has claimed she used to stand over their sleeping sons with a knife in her hand.

Kevin Federline - who split from the Toxic hitmaker in 2007 after three years of marriage - claimed that on more than one occasion their boys Sean Preston, 20, and 19-year-old Jayden James awoke to find their mom watching over them before leaving the room with no explanation.

In an excerpt from his memoir You Thought You Knew shared by the New York Times newspaper, Kevin wrote: "They would awaken sometimes at night to find her standing silently in the doorway, watching them sleep - 'Oh, you're awake?' - with a knife in her hand.

"Then she'd turn around and pad off without explanation."

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