A former partner of Cristiano Ronaldo has come out in support of the woman accusing him of rape.

Jasmine Lennard, who is currently starring in Celebrity Big Brother UK, lashed out at the star footballer, calling him a "f***ing psychopath".

The pair dated 10 years ago, but the Sun reported the two had been in communication again for the last 18 months.

Lennard revealed she would reach out to Ronaldo's accuser, Kathryn Mayorga, on Twitter after watching the Surviving R. Kelly documentary series in which women reveal their stories of abuse at the hands of the R&B singer.

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"No one has any idea what he is truly like. If they had half a clue they'd be horrified," she said in one of a series of tweets.

Lennard alleged the Portuguese star reached out to her to "stay silent and get on board with supporting and defending him" but she was going to "do the right thing".

Ronaldo, now playing for Juventus in Italy, is alleged to have raped aspiring US model Kathryn Mayorga, 34, nine years ago in a Las Vegas hotel room.

Leaked court papers suggest his legal team agreed to pay £287,000 (NZ$540,000) to stop her pursuing charges and gagged her from speaking out — but did not admit any wrongdoing.

"He legitimately thinks he's going to get away with it. Not now. I have had a relationship with him for a decade. We have been communicating pretty much on a daily basis for the last year and half and I have messages and recordings that'll be invaluable to Kathryn and her team in showing his true nature and character how unwell he is. I refuse to sit back any longer. It's wrong. It makes me feel like I'm complicit. I just can't. Reach out to me Kathryn i will help you," Lennard said on Twitter.

Ronaldo, 33, denies the rape claims.