Jamie Lynn's memoir, called Things I Should Have Said, was released this week with the author also undertaking a press tour.

Speaking to ABC News in the US last week, Jamie Lynn Spears said her sister's behaviour had been "erratic", "paranoid" and "spiralling" in the last few years - with Britney hitting back afterwards, accusing her sister of making the comments to sell her book.

Now Britney's conservatorship is over - what happens next?

In a new letter, obtained by US publication Page 6, Britney's lawyer Mathew Rosengart says that Jamie Lynn's book contains "misleading or outrageous" claims about the singer, and that she has "exploited her for monetary gain".

It went on: "Although Britney has not read and does not intend to read your book, she and millions of her fans were shocked to see how you have exploited her for monetary gain. She will not tolerate it, nor should she.

"You of all people know the abuse and wrongdoing Britney had to endure during the conservatorship, after initially growing up with a 'ruinous,' alcoholic father. In fact, your own book reportedly states that your father 'spent most of my life in that cycle of ruinous behaviour. His bouts of drinking caused me periods of torment and sorrow'.

"As I have previously stated, having endured a 13-year conservatorship that stripped her of civil rights and fundamental liberties, Britney will no longer be bullied by her father or anyone else.

"Britney was the family's breadwinner and she also otherwise supported you. Publicly airing false or fantastical grievances is wrong, especially when designed to sell books. It is also potentially unlawful and defamatory.

"Michelle Obama famously said, 'when they go low, we go high' and to Britney's great credit, that is exactly what Britney is going to do, for the time being.

"You recently reportedly stated that the book was 'not about her.' She takes you at your word and we, therefore, demand that you cease and desist from referencing Britney derogatorily during your promotional campaign.

"If you fail to do so or defame her, Britney will be forced to consider and take all appropriate legal action."

The letter is an escalation in their public feud, which began during Britney's court testimony last summer, where she said that she wanted to sue her family for their behaviour during her now-ended conservatorship.

Britney also wrote on Instagram on Tuesday night that she should have "slapped" Jamie Lynn "and Mamma right across" their "f****** faces" when they were younger.

After Jamie Lynn's appearance on ABC News, Britney said she was offended by her sister's comments, saying: "She was never around me much 15 years ago at that time... so why are we even talking about that unless she wants to sell a book at my expense?"

She also accused Jamie Lynn of being "pampered" but later admitted that she loved Jamie Lynn "unconditionally".

Jamie Lynn has not yet responded to the letter.