Warning: This post describes accounts of sexual and physical child abuse.

Happy Days actor Scott Baio has been accused of child abuse by two of his former co-stars on the set of ’80s sitcom Charles in Charge.

Nicole Eggert and Alexander Polinsky, who played siblings Jamie and Adam Powell in the hit show, allege Baio sexually, physically and verbally assaulted and harassed them when they were as young as 14 and 11. Baio, who played their on-screen nanny, was in his 20s at the time.

Eggert alleges the abuse began when she was 14 years old and Baio penetrated her with his finger.

"I was very young and it was shocking, a little," Eggert told NBC News' Megyn Kelly Today last week.

"I had never experienced anything like that before either. So, he was playing not only on my emotions, but on my hormones and all of those things."

Eggert said the abuse continued "often" from that point.

"More than 10 times, more like probably once a week. And then on set, there was just a lot of groping, a lot of fondling, a lot of pulling me on his lap, trying to sneak kisses in the back, which other cast members saw."

She also said before her 18th birthday - the legal age of unrestricted sexual consent in most US states - the pair had sex.

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Like so many victims of abuse, Eggert said she felt too "ashamed" to speak up earlier.

"What my truth is, is that I wasn’t ready to tell my story and for me it was always protecting the show and protecting that whole legacy that nothing happened there. That was my delusion and that was my cover-up always."

Baio, now 57, has denied these accusations.

On Wednesday, Eggert's on-screen brother Polinsky made his own claims about the Joanie Loves Chachi star at a press conference.

Polinsky said the abuse began when he was 11 years old and walked in on what he thought was an innocent exchange between Baio and Eggert, but what he now thinks was an episode of sexual abuse.

"I walked in on them together behind the set. Nicole was on Scott's lap and he didn't appreciate my intrusion," he explained.

"I was so naive, I innocently hopped on his lap expecting to hear a story about Fonzie or Happy Days.

"Scott Baio was a hero to me. Instead of telling us a story, Scott immediately threw me off him and began angrily calling me 'faggot' while Nicole laughed."

What followed, Polinsky alleged, was "a pattern of abuse that was unrelenting" over the several years the show was filmed. He believed this was some sort of retaliation for what he had seen.

"Scott pulled down my pants in front of over 100 people," Polinsky said.

"Another time, he laughed as he cut a hole in the canvas wall of my private dressing room on set and exposed his genitals to me."

Polinsky said he believed adults were aware of what was happening, but remained silent in fear of losing their jobs.

Baio's lawyer Jennifer McGrath also gave a press conference on Wednesday, denying all of the pair's allegations and threatening to sue them.

"Scott denies every single claim of inappropriate behaviour. Almost every one of them can be refuted with evidence," she said.

Baio also denied the accusations in a Facebook post, where he focussed on Eggert's claim he had sex with her when she was underage.