Britney Spears' lawyer is going scorched earth on Kevin Federline ... ripping him for yapping about her relationship with their boys and posting videos of family matters.
Attorney Mathew Rosengart says ... K-Fed's decision to post old footage of Britney arguing with boys Sean Preston and Jayden Jameswas cruel, bottom-the-barrel behavior.
Britney's lawyer says in doing so, K-Fed violated the singer's privacy and dignity, undermined the children, demeaned himself as a father, and violated societal norms ... and now Rosengart is threatening legal action, suggesting a criminal cyber-bullying statute may have been violated.
Remember ... Britney and K-Fed are going back and forth in public over her relationship with the kids, with Kevin posting the videos and giving an interview to DailyMail, while Britney fights back with social media posts.
Rosengart says he's not about to let Britney be bullied like she was during her conservatorship ... and tells us he's working with Instagram to turn up the heat on K-Fed.
Britney's attorney says Kevin should keep their family matters private. In fairness, Britney also went public about her children Wednesday in her own IG post, calling the kids "hateful," and suggesting K-Fed's house is a marijuana den.
Bottom line for Britney's camp ... Rosengart says she's beloved and respected by millions, which can't be said about K-Fed.