Britney Spears' ex-husband has claimed she used to stand over their sleeping sons with a knife in her hand

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."

Elsewhere in the book, Kevin spoke of his fear that "something bad" is going to happen to his troubled former wife.

He wrote: "The truth is, this situation with Britney feels like it's racing toward something irreversible.

"It's become impossible to pretend everything's OK.

"From where I sit, the clock is ticking, and we're getting close to the 11th hour. 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."

Kevin went on to express his concern that the Free Britney movement - which campaigned to have his ex-wife's conservatorship terminated - may have "started from a good place" but has now left professionals who could help the Gimme More hitmaker afraid to step in.

He wrote: "All those people who put so much effort into that should now put the same energy into the 'Save Britney' movement. Because this is no longer about freedom. It's about survival."

While Kevin doesn't know the perfect "solution", he appealed for anyone "who has ever been moved by Britney" to stand by his children and their mother.

He wrote: "Now, more than ever, they need your support. I've been their buffer for years, but now it's bigger than me. It's time to sound the alarm."

The 47-year-old dancer is keen for his sons to have an "incredible relationship" with their sons.

He told the New York Times: "I've never, ever, once, been against Britney.

"I've only tried to help my sons have an incredible relationship with their mother. And it's hard because when I really reflect on everything that's happened - my kids do not know the woman that I married. And I've spent two decades trying to bridge that gap."

A spokesperson for Britney has hit out at Kevin for "profiting off" the star with his book.

The representative told People magazine: "With news from Kevin's book breaking, once again he and others are profiting off her and sadly it comes after child support has ended with Kevin.

"All she cares about are her kids, Sean Preston and Jayden James and their well-being during this sensationalism. She detailed her journey in her memoir [The Woman in Me]."