Britney Spears has threatened to sue her sister, Jamie Lynn Spears, if she continues to make ‘derogatory’ references to the Grammy-winner while promoting her new memoir.

Since the book, ‘Things I Should Have Said’ was released last week, Jamie Lynn has embarked on a press tour to publicise it and has described her sister’s behaviour as having been ‘erratic’, ‘paranoid’ and ‘spiralling’ in recent years.

Britney has taken issue on social media with Jamie Lynn’s recollections, including comments made by her during a recent interview on the ‘Good Morning America’ TV programme. Britney’s lawyer, Mathew Rosengart, has now sent Jamie Lynn a letter demanding that she ‘cease and desist from referencing Britney derogatorily during [her] promotional campaign’.

The letter warns that if Jamie Lynn fails to stop referencing Britney in a derogatory manner or defames her, ‘Britney will be forced to consider and take all appropriate legal action’.

Rosengart says that although Britney has not read Jamie Lynn’s book, it contains ‘misleading or outrageous’ claims about the pop icon, and has ‘exploited her for monetary gain’.

The letter also warns that ‘publicly airing false or fantastical grievances is wrong, especially when designed to sell books. It is also potentially unlawful and defamatory’.

Jamie Lynn has yet to comment on the cease-and-desist letter. During the ‘Good Morning America’ interview, Jamie Lynn said that she had ‘always been my sister’s biggest supporter’, and had taken steps to help Britney end the controversial conservatorship that was terminated late last year.