EXCLUSIVE

'RHOC' stars Tamra Judge and Shannon Beador have been running their mouths and spreading vicious lies about Alexis Bellino's estranged husband ... according to him, and he's threatening to sue.

TMZ has learned Jim Bellino's lawyer fired off letters to both women -- and in one, he rips Tamra for a recent interview where she called him a "shady mother****er" who's going to jail. He also says she defamed Jim by suggesting his divorce might be "fake and for legal reasons."

TMZ broke the story ... Jim filed for divorce from Alexis last week after 12 years of marriage.

In the letter to Shannon, obtained by TMZ, he says she flapped her gums on a podcast and lied about him shutting down his trampoline biz. Even worse, he says she falsely stated, "people get paralyzed ... apparently that happened" -- again, referring to his company.

Jim says both women are not only damaging him with their "morally corrupt" gossip, but could potentially harm his children.