Britney Spears is not backing down in the heated public feud with her sister, as she calls for Jamie Lynn to take a lie detector test over claims she has made about her in a new book.

Despite legal letters flying either side, the Toxic hit-maker, 40, once again took to social media to make her remarks about her younger sibling, whose new memoir has been hailed as a “national best seller”.

“National best seller ???? DUH,” she remarked sarcastically on Instagram, followed by a rolling eyes emoji.

“The timing of your book was unbelievable Jamie Lynn … especially knowing the whole world had no clue what was really done to me !!!! My whole family including you is saying YOU DIDN’T KNOW …. Bull**it !!!” Britney continued to fume.

“I wish you would take a lie detector test so all these masses of people see you’re lying through your teeth about me !!!!

“I wish the almighty, Lord would could come down and show this whole world that you’re lying and making money off of me !!!! You are scum, Jamie Lynn..,” she concluded.

Earlier this month, Britney sent a cease and desist letter to her sister over her memoir, Things I Should Have Said.

The legal note accused Jamie Lynn, 30, of exploiting Britney, reading: “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.”

Jamie Lynn fired back with her own legal letter, with her lawyer writing: “Your letter claims that Jamie Lynn’s book is potentially unlawful, defamatory, misleading and has outrageous claims in it. Yet, you admit that neither you nor your client have actually read any portion of the book.”

He pointed out that it is impossible to understand what Britney is requesting his client to cease and desist from doing since the letter fails to specify any legal wrongdoing whatsoever.

The letter also accused the star of “cyberbullying” and called her actions “vile”.

Jamie Lynn’s book comes two months after Britney was freed from a controversial conservatorship which saw her personal, financial and business decisions controlled for 13 years.