Britney Spears’ war of words with her sister just escalated, as the pop star is now threatening legal action. 

Spears’ attorney, Mathew Rosengart, has sent a cease and desist letter to Jamie Lynn Spears, demanding she stop “referencing Britney derogatorily” while promoting her new book and warning that Britney will consider “appropriate legal action” should she “fail to do so or defame her,” Variety reports. 

Jamie Lynn Spears has recently been promoting her new book Things I Should Have Said, which she discussed on Good Morning America last week. She told ABC she tried to help her sister amid her conservatorship battle, and she has also discussed an incident in which Britney allegedly locked them in a room with a knife. In her book, Jamie Lynn Spears reportedly describes her sister’s behavior as “erratic.” 

Britney Spears fired back on social media, alleging her sister was trying “to sell a book at my expense,” and she denied her account of the knife incident. “Only a scum person would make up such things about someone,” Britney Spears wrote. On Tuesday, the “Toxic” singer also said on Instagram that she wishes she had “slapped” Jamie Lynn Spears and their mother “right across your f—ing faces.” 

Rosengart in the letter to Jamie Lynn Spears slams the “misleading or outrageous claims” in her “ill-timed book,” warning that Britney “will not tolerate” being exploited “for monetary gain,” per Variety. “Publicly airing false or fantastical grievances is wrong, especially when designed to sell books,” the attorney writes. “It is also potentially unlawful and defamatory.” 

Last week, Jamie Lynn Spears responded to Britney Spears’ posts about her, writing that she has been receiving death threats because of the “vague and accusatory” statements. She also said that their private conversations “don’t match what you post on social media” and that the book “is not about” Britney, adding, “I will always love my big sister, and be here for her.”