Recalling the night of the assault, Allen told The Next Episode she had been at a party with the unidentified male in the Caribbean.
"We got to my hotel. I couldn't find my room keys. So he was like, 'Well, why don't you sleep in my bed while I go and get the keys or whatever.' So I passed out in his bed.
"I woke up and he was in my bed naked slapping my bum."
She said she could feel him trying to have sex with her.
"I made a decision, I didn't want to go to the police. I didn't want to make a fuss and I wanted to keep it quiet.
"I remember thinking about his mum and how she would deal with the news that her son was a sexual predator and I was prioritising everybody else in this situation except for myself."
Allen explained that it was the #MeToo movement, when women started coming forward with allegations of sexual abuse within the film industry, that gave her the courage to first share her horrific experience in her book My Thoughts Exactly published last year.
Following the book's release, she said she met with Max Lousada, a chief executive at her record label, to make a more formal complaint in order to protect others in the industry.
She said she believed her alleged attacker worked with other young female artists, so she felt a certain responsibility to speak up.
"I would feel awful if I found out that somebody much younger and more vulnerable had had a similar experience that could have been prevented," she said.
"It's my responsibility just to let some people know that this incident happened," she said.
Warner Music said it was "appalled" by the allegations, adding: "We're very focused on enforcing our Code of Conduct and providing a safe and professional environment at all times."
Allen first signed with Warner in 2013, and is currently working on her last album for the label.
