In a joint letter released by 25 cast and crew members of the show, Schwahn has been accused of "repeated unwanted sexual harassment of multiple female members of cast and crew", according to The Hollywood Reporter.
They said their counterparts at "One Tree Hill" motivated them to tell their own stories of harassment.
"Despite hearing rumours about his behaviour on One Tree Hill, those of us involved from the early stages went into the filming of the pilot hoping they were just that rumours. By the time we wrapped this had irrevocably proven not to be the case.
The letter has been signed by Hatty Preston, Sophie Colquhoun, Alex Watherson, Lydia Rose Bewley, April Church, Annalise Beusnel, Poppy Corby-Tuech, Florence Chow, Charlie Jones, Isabella Artitzone, Jade Armstrong, Rachel Walsh, Tania Vernava among others.
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"Whilst voraciously condemning the actions of one man, we would like to make note of our gratitude to others. Thank you to so many of the Royal men who ensured we were never alone in social situations with him and took two steps towards us for his every one," it said.
"E!, Universal Cable Productions and Lionsgate Television take sexual harassment allegations very seriously, investigate them thoroughly and independently, and take appropriate action. Lionsgate has suspended Mark Schwahn from 'The Royals' as we continue our investigation," the companies had said in a statement to Variety.
Schawhn was first accused by the cast and crew of the 2003 hit show "One Tree Hill", who had released a joint statement in support of writer Audrey Wauchope after she accused the 51-year-old creator of harassment without naming him.