Their sizzling on-screen sexual attraction was obvious to millions of fans of the classic romantic adventure Romancing The Stone.

But for years Michael Douglas, the very much married star of the film, denied rumours that he and co-star Kathleen Turner had enjoyed a torrid full-blown affair on set.

Turner, who was single but dating real estate mogul Jay Weiss – the man she would later marry – steadfastly denied the affair, too, even when Hollywood gossip reached fever pitch when Douglas’s then wife Diandra became so worried that she flew to the Mexican set and confronted her.

But now, four decades on from the 1984 blockbuster, Douglas, 82, who has been happily married to Catherine Zeta-Jones since 2000, has ‘come clean’.

In his explosive memoir One Helluva Ride, he admits he and Turner, now 72, fell madly in ‘lust’ at first sight: ‘Kathleen was smart, funny and sexy, with a deep sultry voice that became her signature. I instantly liked her and we were immediately attracted to each other.’

Douglas had been married to model Diandra Luker since 1977 and had a young son, Cameron. The Fatal Attraction actor added: ‘Things at home were in a bad place. Diandra and I were both having extramarital affairs. The truth is that during the production of Romancing The Stone, Kathleen and I fell hard for each other and began a relationship.

‘It wasn’t a fly-by night affair; it grew naturally from how closely we were working together, how much pressure we were under, how much we respected and cared for one another, and how attracted we were to each other.

‘Like every other guy in the world at the time, I was knocked sideways by Kathleen.’

American actors Michael Douglas and Kathleen Turner on the set of Romancing the Stone directed by Robert Zemeckis

In his explosive memoir One Helluva Ride, Douglas admits he and Turner fell madly in ‘lust’ at first sight (pictured at Toraille Waterfall in the film Romancing the Stone)

The pair kept their affair hidden from everyone including their co-star Danny DeVito and director Robert Zemeckis.

Douglas said: ‘We found each other first as friends, then as co-stars, and finally as lovers.

‘Following an exhausting day of shooting in the jungle, we’d go out and have a meal, or head out with the group and afterward I would sneak into her hotel room after everybody was asleep.’

Their clandestine romance ended when Douglas’s wife flew from LA to Mexico and brought their young son.

He said: ‘Diandra grew suspicious. A woman’s intuition, I suppose.’

Their marriage limped on until 2000 when the couple divorced – with Douglas marrying Ms Zeta-Jones that same year.

He and Turner had a ‘gentleman’s agreement’ never to discuss their relationship and for four decades both stuck to that pact.

Douglas wrote: ‘The official story went as follows: Yes, we confessed, we had amazing chemistry. Yes, we were both, as Kathleen said, ‘terrible flirts’, and yes, we were both tempted, but nothing ever happened between us.

‘That was a lie. It was a lie meant to protect my wife and son, a lie we told the world for 40 years.’

Turner honoured that pact in an interview with The Mail on Sunday a few years ago, saying: ‘The chemistry was there and Michael and I were both terrible flirts. We were in the jungle and I was feeling very romantic towards him. It was going to happen.

‘Then I walked on set one day and his wife was suddenly there. She sat next to him and that was that. You don’t mess with another woman’s husband.’

When asked last night how she feels about Douglas ‘outing’ their illicit romance, Turner appeared far from upset: ‘Good heavens, what a fuss about something 40 years ago! I love Michael and I look forward to enjoying the book.’

Last night, a Hollywood producer who worked with Douglas said: ‘It was always one of Hollywood’s greatest rumours and now it’s been proven to be correct.

‘It took 40 years for him to admit it but you only have to watch the movie to know the sexual attraction was real.’