Polanski is suing Condé Nast, the publisher of Vanity Fair magazine, for an article in which he was accused of propositioning a woman on the way to the funeral of his murdered wife, the actress Sharon Tate, 35 years ago.

The case is due to start this month. Polanski will give evidence by video link because he has been told that he could be arrested and charged with rape if he came to Britain.

Farrow, the former wife of Frank Sinatra who later had children by André Previn and Woody Allen, is potentially the most important witness in a gallery of stars that is being lined up to support Polanski.

She was present with Polanski at Elaine’s, a fashionable restaurant in New York, where Vanity Fair claimed that the incident took place. She refers to the evening in her autobiography, published in 1997.

Farrow has told associates in Hollywood that she has set aside two days this month to fly to London to testify on behalf of Polanski. “Mia regards Roman as a close friend and she says she will do all she can to help him,” an insider said.

Farrow rose to fame in Polanski’s 1968 film Rosemary’s Baby, playing a woman impregnated by the devil.

Polanski, 72, has told his lawyers that he was “utterly distraught” over the magazine’s claims that he propositioned a Swedish woman, who was a complete stranger, on his way to Tate’s funeral in 1969. Tate was stabbed to death by followers of Charles Manson, a cult leader and convicted serial killer. She was eight months pregnant with Polanski’s child when she and four visiting friends were murdered in her home in Bel Air, California.

Polanski was abroad when his wife was killed and was devastated by her death. He is angered that the Vanity Fair article portrays him as being disloyal to his wife’s memory.

Polanski was once feted in Hollywood, but his career was damaged by a series of sexual indiscretions. He became a fugitive from American justice in 1977 when he was charged with the statutory rape of Samantha Geimer, a would-be model, who was then aged 13.

The incident took place during a photo shoot at the home of Jack Nicholson, the actor. Polanski agreed to plead guilty. He was on bail awaiting sentence when he decided to flee the country and has not returned to America or Britain since.

Geimer, now a 40-year-old mother of three, has publicly forgiven Polanski. Prosecutors say he will definitely have to spend time in prison for the rape charge if he ever returns.

Polanski partly redeemed his reputation two years ago with the making of the Holocaust film The Pianist, which won three Oscars. The film drew on the traumas he had suffered when, as a boy of nine, he escaped from the Jewish ghetto in Cracow.

Polanski’s High Court libel case is likely to confirm London’s reputation as the libel capital of the world. Vanity Fair is an American magazine, but Polanski is not suing there because the country has more restricted libel laws. American plaintiffs have to prove “actual malice” in order to succeed.

Fighting the case in Britain was a risk for Polanski. Schillings, his lawyers, had warned him that appearing in person might lead to his immediate arrest and his eventual extradition to America.

Editors at Vanity Fair had hoped that Polanski would pull out of the case after the Court of Appeal overturned a ruling by the trial judge that allowed Polanski to testify by video link. But four months ago the law lords ruled by a 3-2 majority that he could give evidence from Paris. He will become the first person to bring a contested libel claim in an English court by giving evidence via a video-link.

Now a French citizen, he lives in Paris with his wife, the French actress Emmanuelle Seigner, who he met when he was 53 and she was 18. They have two children.

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