Rumours of a rift between the siblings began last year and were confirmed by the 30-year-old actress during a TV interview last week, with the appearance sparking a public back-and-forth between the sisters over social media.

Now, the pop star's lawyer Mathew Rosengart has got involved, issuing a cease and desist letter to the Sweet Magnolias star to ask her to stop referring to Britney "derogatorily" during her book's promo campaign.

"Dear Ms. Spears: As you know, I represent your sister Britney Spears, and I write at her request concerning the above-referenced matter," the letter, obtained by Page Six, begins. "We write with some hesitation because the last thing Britney wants is to bring more attention to your ill-timed book and its misleading or outrageous claims about her.

"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."

The letter mentions Britney's recently terminated conservatorship, which lasted 13 years and was initially put in place by their father, Jamie Spears.

"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," the letter continues, quoting from the actress' memoir. "In fact, your own book reportedly states that your father 'spent most of my life in that cycle of ruinous behavior.'"

The attorney states that the Gimme More singer "will no longer be bullied by her father or anyone else" after enduring the legal arrangement, which "stripped her of civil rights and fundamental liberties". He also accused Jamie Lynn of "airing false or fantastical grievances" to gain publicity for the memoir.

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

Things I Should Have Said was released on Tuesday.