Opri to Birkhead -- Take it Back or I'll Sue!

TMZ has obtained dueling lawyer letters between Larry Birkhead and Debra Opri and THIS JUST IN: It's gotten really ugly.

Opri is pissed that Larry B appeared on Larry King's show this week and claimed he witnessed Opri feed newly-minted author Rita Cosby confidential documents in his custody battle. Opri's lawyer, David Owen, fired off a letter obtained by TMZ, in which he says, "That statement is false and defamatory....we are giving Mr. Birkhead an opportunity to issue a retraction."

Not gonna happen. Birkhead's lawyer, Micheal Trope, fired off his own letter -- also obtained by TMZ -- in which he says, "Mr. Birkhead declines your invitation to issue any type of retraction," adding, "...truth is a defense."

Trope ends with this pretty funny salvo: Larry would "like her to avoid the expense of hiring a process server to deliver the complaint. I have Mr. Birkhead's authority to accept service."

In this case, every penny counts.