Ronan Farrow believes his father Woody Allen sexually abused his sister Dylan Farrow when she was a child.

Woody Allen with Blake Lively and Kristen Stewart at Cannes

Woody Allen with Blake Lively and Kristen Stewart at Cannes

The 27-year-old journalist has penned an essay for The Hollywood Reporter (TheHollywoodReporter.com) titled 'My Father, Woody Allen and the Danger of Questions Unasked' in which he publicly gives his older sibling his full support over her allegation the film director sexually assaulted her when she was just seven years old.

Ronan states he witnessed "inappropriate" behaviour from his father, writing: "I believe my sister. This was always true as a brother who trusted her, and - even at five years old - was troubled by our father's strange behaviour around her: climbing into her bed in the middle of the night, forcing her to suck his thumb - behaviour that had prompted him to enter into therapy focused on his inappropriate conduct with children prior to the allegations."

Ronan's comments in the article follow an open letter his Dylan, now 30, wrote about Woody in 2014, claiming he abused her in a "dim, closet-like attic" at their family home.

Their mother, 'Hannah and Her Sisters' actress Mia Farrow, never pressed charges against Woody, which Ronan believes was a decision she made to try and protect Dylan.

Ronan wrote: "My mother still feels it was the only choice she could make to protect her daughter. But it is ironic: my mother's decision to place Dylan's wellbeing above all else became a means for Woody Allen to smear them both."

Ronan is also dismayed that after Dylan made her abuse allegations against Woody, media company Amazon continued to finance Woody's ventures.

The 'Blue Jasmine' writer-and-director is developing a television series for Amazon which will star Miley Cyrus, and the company have purchased the rights for his latest movie 'Cafe Society' for $15 million.

Ronan said: "The old school media's slow evolution has helped to create a culture of impunity and silence. Amazon paid millions to work with Woody Allen, bankrolling a new series and film."

He is also dismayed that major stars such as Kristen Stewart, Blake Lively and Steve Carell all continue to work with Woody, despite the allegations made against him.

He said: "Actors, including some I admire greatly, continue to line up to star in his movies. 'It's not personal,' one once told me. But it hurts my sister every time one of her heroes like Louis C.K., or a star her age, like Miley Cyrus, works with Woody Allen. Personal is exactly what it is - for my sister, and for women everywhere with allegations of sexual assault that have never been vindicated by a conviction.

"He'll have his stars at his side - Kristen Stewart, Blake Lively, Steve Carell, Jesse Eisenberg. They can trust that the press won't ask them the tough questions. It's not the time, it's not the place, it's just not done."

Woody, 80, appeared at the Cannes Film Festival on Wednesday (11.05.16) flanked by Kristen and Blake for a photocall for his new film which stars the two actresses.

Woody and Mia have had an embittered relationship ever since the Hollywood legend left her and married Soon-Yi Previn - the daughter Mia adopted with her ex-husband Andre Previn - in 1997.

Woody and Soon-Yi have two children together, daughters Bechet, 17, and 16-year-old Manzie.

Ronan regrets that he didn't speak up about his sister's allegations sooner and admitted it was because he was afraid it would jeopardise his career, much to his shame now.

He added: "(Her) decision to step forward came shortly after I began work on a book and a television series. It was the last association I wanted. Initially, I begged my sister not to go public again and to avoid speaking to reporters about it. I'm ashamed of that, too. With sexual assault, anything's easier than facing it in full, saying all of it, facing all of the consequences. Even now, I hesitated before agreeing to The Hollywood Reporter's invitation to write this piece, knowing it could trigger another round of character assassination against my sister, my mother or me. When Dylan explained her agony in the wake of powerful voices sweeping aside her allegations, the press often willing to be taken along for the ride, and the fears she held for young girls potentially being exposed to a predator, I ultimately knew she was right."