Former child actor and 'Oliver!' star Mark said he donated sperm to the late 'King Of Pop' in 1996 and is willing to take a paternity test to prove he is Paris' natural father.Â
He said: "It's been a secret for so long. Michael asked me if I would give him my sperm and I said yes.Â
"It was a gift to him, no money was paid, it was something I was honoured to do. He wanted children so badly."
Mark â who is godfather to Michael's three children: Prince Michael, 12, Paris, 11, and Prince Michael II, also known as Blanket, seven â made the claims after noticing the resemblance between Paris and his own daughter, Harriet, 15.
Until now he had kept his sperm donation secret from even his own wife and family, but has come forward after fearing he will be shut out of the children's lives.Â
Mark added to Britain's News Of The World newspaper: "With Michael's mother, Katherine, now their legal guardian it's like the kids are being isolated.Â
"Up until Michael's memorial I was in touch with them on a weekly basis.
"I'm the Godparent to them and Michael was Godparent to all my four kids.Â
Our two families spent a lot of time together, and had a lot fun together. Now I'm not able to have any communication with the children. This isn't what Michael would have wanted."
Mark â now an osteopath and acupuncture specialist â first bonded with Michael in 1981 over the fact they had both been child stars, with Michael singing in the Jackson 5 and Mark starring in 1968 Oscar winning film 'Oliver!'Â
He added he believed Michael was too shy to have sex with anyone, and explained how he came to offer his sperm.Â
Mark, 51, said: "Michael found it very difficult to have relationships with women. He found the sexual act something he couldn't do.
"Michael wasn't gay. But I'd say he was asexual. He was very conscious of his own body.
"In one of our regular phone conversations he said he realised having kids was a natural thing but he just couldn't do it.
"I think he'd already tried using his own sperm for artificial insemination but it hadn't worked for him, so I made an off-the-cuff comment saying: 'Try mine!'
"There was this silence on the phone for a moment. Then Michael said, 'Did you mean that?'
"I didn't expect it to happen, but I accepted. It's something I wouldn't do for anyone else. But I wanted to do it for him. I was honoured to do it."