The ex-husband of pop star Britney Spears gives new details on the pair's tense relationship in his new memoir You Thought You Knew.

Kevin Federline, her one-time backing dancer, presents a picture of Spears's allegedly erratic behaviour as a wife and mother, and says he continues to have concerns for her wellbeing.

Spears has spoken out against the book, which will be published on Tuesday, and accuses Federline of trying to profit from his association with her.

What does the book say about Spears?

Federline makes a number of allegations against Spears, in the memoir shared with the BBC ahead of the book's release by publisher Listenin.

He accuses Spears of unhealthy behaviour as a mother, including drinking when she was pregnant and taking cocaine while her two sons were still breastfeeding. Spears has previously denied having substance abuse issues.

When they were teenagers, the couple's sons were fearful of Spears, Federline also alleges. "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," he writes. "Then she'd turn around and pad off without explanation."

Federline also expresses his concern for his estranged ex-wife, writing: "The truth is, this situation with Britney feels like it's racing toward something irreversible. From where I sit, the clock is ticking, and we're getting close to the 11th hour."

He writes that he was supportive of a conservatorship, under which many aspects of Spears's life were controlled by others for years. Of the Free Britney movement, Federline writes that he could "never fully get behind it".

What's the history of their relationship?

The pair got together in the summer of 2004, after Spears's commercial peak.

They married a few months later and quickly welcomed their two sons: Preston, now 20, and Jayden, now 19.

Spears filed for divorce in 2006 and the divorce was finalised in 2007.

The couple fought publicly over custody of their sons and ultimately Federline was given sole custody of the boys.

In the wake of Federline getting full custody, Spears was seen behaving in a way that Federline characterises in his book as erratic.

She made headlines with incidents like shaving her head and hitting a photographer's car with an umbrella.

Those public incidents raised concern about her mental welfare, and after she was taken to hospital twice, a temporary conservatorship was established in 2008 and made permanent later that year.

Spears remained under that controversial arrangement until 2021, under which her father and others controlled her finances and aspects of her personal life. She did not regain custody of her children until after it was lifted.

What has Spears said?

Spears has accused Federline of attacking her during his recent press appearances to promote the book. She described the contents of the memoir as "extremely hurtful and exhausting", but suggested the work would sell better than her own memoir had done.

In that book, The Woman in Me, Spears accused her ex-husband of trying to "convince everyone I was completely out of control".

Spears has also defended her relationship with her sons. "I have always pleaded and screamed to have a life with my boys," she wrote on social media on 15 October, adding that "relationships with teenage boys is complex".

In a statement to US media, a Spears spokesperson added: "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 BBC has contacted Federline's lawyer for comment on Spears's counter-accusations.

Why is this happening now?

While the pair dominated headlines during their relationship and subsequent divorce, Federline in particular has largely remained out of the limelight in recent years. And recently, the spotlight on Spears has largely been over her social media postings.

Federline has said he considered writing a book for many years but wanted to wait until his children were older to complete it.

"I picked it up and put it down quite a lot over probably a five-year period," he told the Associated Press. "I think that it's a very good description of me, who I am, the father I've become, the husband I am, the ex-husband I am."

He also told AP: "I want my children to be able to move forward in their lives and know that the actual truth of everything is out there."