The cable network, along with the production companies behind The Royals, announced the news on Wednesday.
"E!, Universal Cable Productions and Lionsgate Television take sexual harassment allegations very seriously, investigate them thoroughly and independently, and take appropriate action," the companies said in a statement released to ET. "Lionsgate has suspended Mark Schwahn from The Royals as we continue our investigation."
The allegations leveled against Schwahn originate from his time as the creator and showrunner of the CW drama One Tree Hill.
"Many of us were, to varying degrees, manipulated psychologically and emotionally," the joint letter claimed. "More than one of us is still in treatment for post-traumatic stress. Many of us were put in uncomfortable positions and had to swiftly learn to fight back, sometimes physically, because it was made clear to us that the supervisors in the room were not the protectors they were supposed to be."
"Many of us were spoken to in ways that ran the spectrum from deeply upsetting, to traumatizing, to downright illegal," the letter continued. "And a few of us were put in positions where we felt physically unsafe. More than one woman on our show had her career trajectory threatened."
The letter came two days after former One Tree Hill staff writer Audrey Wauchope took to Twitter over the weekend to accuse the showrunner -- whom she did not mention by name -- of sexual harassment during her time on the show.
As for Schwahn's E! drama, production on the fourth season of The Royals concluded in September and the season is still set to air next year, as planned.