In his new memoir One Helluva Ride the legendary actor, 82, admitted that the pair had an affair during the making of their 1984 classic Romancing the Stone.

At the time the star was married to his first wife Diandra Luker and Kathleen was dating her eventual husband, real estate mogul Jay Weiss. 

And now Michael has shared his advice for the next generation.  

'A lot of actors make the mistake early in their career of having a relationship with their leading lady,' he said at the Royal Festival Hall in London. 

'What they forget is that you can break up. They're having to perform these love scenes, and they hate each other.

'We've all made that mistake, and I try to warn people.' 

During filming for Romancing the Stone, rumours of their romance reached such a crescendo that Diandra travelled to the Mexican set of the film to confront Kathleen, who always denied the affair publicly.

However, in his memoir last month, Michael admitted to the dalliance, detailing in his book how he and Kathleen became swept up in 'lust' on the set of the adventure film.

'Kathleen was smart, funny and sexy, with a deep sultry voice that became her signature,' he wrote. 'I instantly liked her and we were immediately attracted to each other.'

His home life, he maintained, was '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.'

Far from being 'a fly-by night affair', their romance '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.'

They were apparently able to conceal the runaround from their colleagues, up to and including their co-star Danny DeVito and director Robert Zemeckis.

Photos։ Jas Lehal/Assignaments/Press Association, Everett/Shutterstock