Britney Spears Wishes She Slapped Mom and Jamie Lynn's 'F***ing Faces'

And just like that ... the truce between Britney Spears and Jamie Lynn Spears is over, 'cause Brit's now openly wishing she'd slapped some sense into her sister and their mom when she had the chance.

Jamie Lynn's new book, interviews and social media posts continue to trigger angry reactions from Britney -- and in the latest one she laments not being "strong enough to do what should have been done" to Jamie Lynn and Lynne.

In the post, speaking directly to her kid sis, Britney says she should've "slapped you and Mamma right across your f***ing faces!!!!!"

Britney's rant is basically about how, in her view, Jamie Lynn was a spoiled younger sibling riding Britney's coattails and living the good life at home while Britney worked her butt off to make tons of money for her family.

BS also accuses her mom of being high on pain meds after splitting from her dad, Jamie Spears, claiming Lynne "was more messed up than anything !!!!"

As you know ... Britney's been taking shots at Jamie Lynn over claims her little sister makes about life with Britney in the book, "Things I Should Have Said."