Michael Douglas Reveals He and Kathleen Turner Lied About Romance For Decades

Michael Douglas is getting a 40+ year lie off his chest ... admitting he romanced Kathleen Turner's stone while shooting a famous movie together in the 1980s.

The actor revealed the relationship in his new memoir "One Helluva Ride" ... claiming he and his then-wife, Diandra Luker, were essentially separated when he went down to Mexico to shoot the 1984 smash hit "Romancing the Stone."

During filming, Michael says he and Kathleen became close -- even though she apparently wasn't the first choice to play his love interest -- and eventually "fell hard for each other and began a relationship."

Michael says they kept the relationship a secret on set ... going out to dinner after shooting and then sneaking into her hotel room late at night to have sex.

He claims their other costar Danny DeVito and director Robert Zemeckis had no idea the two were together ... but he says Diandra picked up on a vibe when she and their son Cameron came down to visit him on set.

Michael says Diandra told Kathleen the pair were still together ... and Kathleen backed off because she didn't want to complicate anything. Michael says he agreed to stay married to Diandra for Cameron's sake -- and the two remained married until 2000.

Of course, "Romancing the Stone" wasn't the only time he and Kathleen worked together ... they also starred in "The Jewel of the Nile" and "The War of the Roses" -- and have said for 42 years that they weren't together.

In fact, just a few years ago, Kathleen claimed their relationship was all sexual tension and no sex ... but Michael tells a different story.

Worth noting ... Michael maintains a strong relationship with Kathleen -- and he says he asked her for permission before telling the world.