Louis Tomlinson's Baby Mama's Dad Demands Takedown of One Direction Fan Sites

The stepfather of Louis Tomlinson's baby mama has a big problem with a One Direction fan whom he claims posts lies about her, Louis and their child.

Briana Jungwirth's stepdad, Brett Clark, sent a letter to the woman who owns a few 1D fan sites accusing her of defaming Briana, Louis and 11-month old Freddie ... and he's threatening a lawsuit.

Clark says the woman insults Briana's hair, calls her a gold digger, and worst of all, re-posted another fan's theory that the real baby daddy is the stepdad.

The woman who runs these 1D fan sites answered Clark's threat by saying he's the harasser. She says she has a right to express her opinion -- First Amendment and all -- so if he wants to sue he can knock himself out.