Britney Spears is outraged at her sister for hawking her book on TV ... a book she says became a best-seller off her back.
Britney said this of Jamie Lynn Spears ... "You are scum" -- posting video of conversations surrounding her sister's new book "Things I Should Have Said" on "The Talk" and "The Real."
Britney posted clips from the shows, where several hosts challenged Jamie Lynn for writing a book rather than talking it out with Britney privately. Britney wrote, "What these women are saying here is pretty clear!!! I'm just kinda shocked that more people like these real soul sisters aren't telling it like it is."
Britney went on ... "The nerve of you to sell a book now and talk s*** but your f***ing lying just like you lied about Alexa Nikolas !!!!"
And, Jamie Lynn's older sis challenged her to take a polygraph test.
Britney has called Jamie Lynn "scum" before, but it looked like the ice may have thawed a little a week or so ago. That's clearly not the case.