He says that after they met in 1991, he was in love with her and "she was in love with me, no question".
And the Hollywood star - now married to Michael Douglas - proved her loyalty to her old flame when a bottle of champagne was delivered to his home two days after he walked free from court last week.
The card with it read: "From Catherine and Michael".
Leslie - cleared of indecent assault charges in the wake of the Ulrika Jonsson rape allegation - met Zeta-Jones in 1991 at a party after a Chesney Hawkes concert.
The former This Morning presenter was then working on Blue Peter, and Zeta-Jones was playing Mariette in the TV show The Darling Buds Of May.
He tells the Daily Express: "It was the real thing alright. I was in love with her and she was in love with me, no question.
"She was lovely, we were together for 18 months, but in the end it didn't work out.
"And no, it wasn't because she wanted marriage and babies and I wasn't ready to commit."
Zeta-Jones, he says - apart from his current partner, Abby Titmuss - provided the most significant relationship of his life.
But he steadfastly refuses to say what went wrong and why they broke up.
"The real reasons are simply not up for public discussion. I simply won't discuss why I broke up with any of my exes," he says.
And there were plenty of exes. Leslie says today: "Over the years I have known a lot of women and taken a lot of women to bed."
He admits he enjoyed three-in-abed sex, watched pornography and bedded a string of women after he found fame as a television presenter.
But he indignantly denies he was ever violent towards them.
Saying he found the rape claims "devastating" and "ghastly" he says: "I am a passionate man and I love passionate sex.
"I may be an oaf, I may be clumsy, but I am not some kind of monster in the bedroom.
"I do not throw women around the room.
"I don't do S & M or bondage. I'm not interested in dressing-up games. I am not kinky."
But he admits he had once "been in a threesome" while he and a girlfriend "shot some home videos for a laugh".
He also says he has watched pornography, but denies having a porn library or watching it on the internet.
Denying he is a Don Juan or even especially good looking, he says: "I am no better with women than your average bloke. I just get more chances.
"Because I have been on the telly for quite a few years there's the recognition, so women want to meet me."
But fame also had its dangers. Apart from the allegations about the rape of Ulrika Jonsson, other women also came forward with accusations of sexual assaults by the Scotsman.
Most of them did so under the cloak of anonymity.
But a few agreed to reveal their identities. Yet their names spark little recognition for Leslie.
"I cannot remember them. I cannot-swear that I have never met them in my life but if I did I just don't remember them."
He knows it might sound strange that a man cannot recall the name of someone he has been intimate with, but Leslie claims it is because he is so sociable.
Large parties were commonplace at his home.
And the number of showbiz parties and corporate hospitality events he attended meant he could not hope to remember all the people he ever met.
"Twenty-odd women went to the press and two to the police, and when it came to charging me, there was one woman. I am no sex fiend. I am no pervert. I have never been those things."
Despite sleeping with numerous female companions, sex was never Leslie's sole aim.
He says: "The real point of having a woman in my bed is having someone to wake up with in the morning. I want to hold and be held. I want intimacy."
Meanwhile Ulrika Jonsson was preparing to leave Britain today as pressure mounts on her to name the man she claims raped her in her autobiography, Honest.
Friends say 35-year-old Ms Jonsson is "devastated" the case against Leslie collapsed last week and is flying to Sweden earlier than planned ahead of her wedding to former Army officer Lance Gerrard-Wright, 35, on 16 August.
She has refused to name her attacker.
But yesterday Leslie challenged his former lover to either name him or clear him of the rape.
Sources close to her were reported today as saying: "Ulrika has been under a tremendous amount of pressure.
"She is veering between floods of tears because she is worried Leslie is going to sue her and anger that once again he is trying to take control of her life.
"She just wants to forget what has been going on in England.
She wants to put the whole Leslie thing behind her and enjoy her wedding to the man she loves."
