Britney Spears has spoken out about "honouring family", following revelations made by her ex-husband Kevin Federline.

In an excerpt from Federline's new memoir You Thought You Knew, the former backup dancer alleged the singer would sit holding a knife as she watched their young sons sleep.

On Tuesday, Spears took to social media to make a pointed remark about remaining true to loved ones as she appeared to hit back at Federline's claims.

Her cryptic Instagram post began with Spears considering the "mystery" of the ocean, before ending with the message: "Honor thy mother and father, but always protect your heart. Only honor them if they have sacredly honored you !!! If not, you find a new family."

Spears and Federline were married from 2004 to 2007, and share sons Sean Preston, 20, and Jayden James, 19.

Hours before the Instagram post, passages from Federline's upcoming book were released.

One excerpt read: "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."

In another extract, Federline claimed "the clock is ticking" as he raised concerns for his ex-wife's health.

He also wrote: "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."

Spears' representative has slammed Federline's allegations and accused him of trying to make money from his relationship with the 43-year-old star.

When asked by Entertainment Tonight if he had chosen to release the book because his child support with Spears has now ended, Federline replied: "I'd be lying if I said that I didn't want this book to be successful. I wish I'll sell a million books or more, or whatever."

The 47-year-old dancer then went on to deny claims that financial gain is behind his motivation to release You Thought You Knew.

He added: "Money is not at the root of this story, (and) it never has been."