In a short message sent on Twitter, Ed wrote: 'I do not know this woman. "He held me down and raped me". She alleged that Westwick, who now stars in White Gold, first suggested that the three of them "should all f-k", which prompted Cohen to say that she was exhausted and wanted to leave.
"I was woken up abruptly by Ed on top of me, his fingers entering my body", she wrote.
"I told him to stop, but he was strong", the Ladies Like Us star wrote.
'And for the longest time, I believed him. "He held me down and raped me".
Ed Westwick has become the latest male star to find himself having to deny, explain or apologize for allegations that he sexually mistreated someone else.
"Like so many women I too have a story of sexual assault, and the outpouring of stories have been both triggering and emotionally exhausting", Cohen wrote.
Cohen claimed she felt uncomfortable in Westwick's company and said she did try to leave, but was told to rest and wait in the guest bedroom.
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'It was a nightmare, ' she continued.
In the post, Cohen wrote that this month has been hard due to the resurfacing of the hashtag #MeToo, which ultimately led her to share her story.
Westwick has responded to the statement with a brief Instagram post, denying the accusation outright. In fact, he says he doesn't even KNOW who she is. Saying there's no way I can go around saying Ed "raped" me and that I don't want to be "that girl".
Cohen noted that she chose to come forward now because she wants to help others realize they are not alone in their suffering.
Cohen added, "I'm sickened to see men like Ed respected in such a public way".
A representative for Westwick did not immediately return HuffPost's request for comment.
Cohen has had minor roles in dozens of shows and films dating back to 2009, while Westwick is best known for playing Chuck Bass in the TV drama "Gossip Girl". He now stars in the BBC Two series White Gold.
The Eathwalkers actress wrote in her post that she struggled with the decision to come forward with her story, but ultimately chose to share it to help others overcome their own struggles.