Ryan Phillippe's ex/alleged baby mama has lawyered up ... and a legal battle is now brewing ... TMZ has learned. 



Alexis Knapp -- the model/actress who claims to be carrying Ryan's baby -- has hired Mark Vincent Kaplan.

Ryan has said he'll take a DNA test once the baby is born and support the child if it's his, but Alexis is not satisfied.

And here's a fun fact.  Kaplan repped K-Fed in his divorce/custody battle with Britney Spears.

Kevin Federline has jumped to his ex-wife's defense, saying claims that Britney Spears beat her children are "baseless."



TMZ broke the story yesterday ... former Britney bodyguard Fernando Flores sued Brit, claiming, among other things,  she "savagely" whipped her two kids with a belt.

Federline now says -- through his attorney Mark Vincent Kaplan -- what TMZ has reported ... that the L.A. County Department of Children and Family Services investigated Flores' allegations and determined they had no merit.

Britney has not responded to the lawsuit, in which Flores also claims she repeatedly sexually harassed him and came on to him in a naked state -- i.e., California.