Joan Collins to Shirley Jones -- You're LYING!!! I'm Not Into 4-Ways

80-year-old Hollywood legend Joan Collins is livid at fellow icon Shirley Jones, insisting she's NOT some porn-watching swinger pervert and never was -- despite claims in Shirley's new book -- and now, she's threatening Ms. Jones.

Sources tell TMZ, Joan -- famous for the '80s soap "Dynasty" -- is pissed over one anecdote in particular, which Shirley recounts in her book "Shirley Jones: A Memoir," a story about the first time Shirley met Joan in the late '60s and a 4-person sex party nearly broke out.

According to the book, Shirley and her then-husband Jack Cassidy were at the home of Joan Collins and Tony Newley for dinner ... and after dinner, Tony proposed they all get naked and watch porn together. Shirley writes, "It was clear what Tony was leading up to -- swinging."

79-year-old Shirley says she turned down the offer -- and now, sources tell us, Joan is accusing her of libel ... insisting the story is bogus.

We're told Joan has sent Shirley a cease and desist letter, demanding her new book be removed from stores everywhere, but Shirley's NOT backing down. She says the story is true and she's standing by it.