BRITNEY Spears lambasted sister Jamie Lynn Spears as "scum" and accused her of "lying" as her sibling's tell-all book was declared a national best seller.

The Baby One More Time popstar, 40, took to her Instagram account to let rip on her younger sister, who recently released memoir Things I Should Have Said.

The pair have become embroiled in a feud since news of the book broke, with Brit threatening to sue Jamie Lynn for derogatory claims before branding her a "brat" for her new tome.

In the latest war of words, mom of two Britney uploaded two clips from TV shows The Talk and The Real, both of which featured panels which took her side,

They argued Jamie Lynn's words would have been best said in private rather than plastered over the new release.

Yet it was in the caption that Brirt showed her true strength of feeling as she raged: "National best seller???? DUH ? …. 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 …. Bulls**t !!

"But what these women are saying here is pretty clear!!!!

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"I’m just kinda shocked that more people like these real soul sisters aren’t telling it like it is!!! Congrats best seller … I’m not surprised at all ????? !!!

"The nerve of you to sell a book now and talk s**t but your f**king lying just like you lied about Alexa Nikolas!!!!

Follow our Britney Spears and Jamie Lynn Spears live blog for the very latest news and updates….

"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!!!!"

The pop princess rounded off her scathing message with the words: "You are scum, Jamie Lynn."

FAMILY FEUD

This marks the latest in a series of events since Britney gained control back of her life last year after her father Jamie's conservatorship officially ended.

Ever since then Britney has been extremely vocal, slamming her family members especially younger sister, Jamie Lynn.

In fact, the performer recently shared she wish she would have "slapped" her sibling as well as her mother Lynne, too

Britney and Jamie Lynn have continued to go back-and-forth amid the release of the Zoey 101 actress' memoir.

Plenty of harsh words and claims have been exchanged, including one allegation that Britney once grabbed a knife and locked her and Jamie Lynn in a room.

While speaking to JuJu Chang on ABC, Jamie Lynn reflected on the alleged incident: "I was scared. That was a moment I had.

"I also was fearful to really say anything because I didn't want to upset anybody, but I also was so upset that she didn't feel safe."

Britney would slam the claims, calling them "crazy lies" and telling her sister she's "stooped to a new low."

The US Sun has since exclusively told how Britney will fight back in the form of releasing her own tell-all book to "set the record straight."

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