Eddie Murphy -- I'm Not Your Daddy -- So BACK OFF!

Eddie Murphy is on the attack against a comedian who has not-so-subtly insinuated Eddie is his daddy.

Eddie's lawyer fired off several threatening letters to Brando Murphy, claiming the comedian has all but advertised on a sandwich board that he's Eddie's offspring. In one letter -- obtained by TMZ -- Eddie's lawyer, legal pit bull Marty Singer, threatens to sue him for millions if he doesn't back off.

According to the letters, these are some of the things Brando has done:

-- Changed his last name to Murphy
-- Performing with Richard Pryor's son, Mason, in the "Sons of Comedy" tour.
-- Showing up at the Coffee Bean & Tea Leaf Eddie frequents and screaming, "My father won't talk to me!"

Brando is not backing down, sending his own threatening letter to Eddie, insisting he's never presented himself as Eddie's son and calling Eddie's accusations "vile, base, contemptible, miserable, wretched and loathsome."

Brando says he'll sue Eddie for defamation if he continues what he calls a campaign of intimidation.