The Polish film director is suing the magazine over a story that alleged he propositioned a Swedish woman in New York as he was on his way to his wife's funeral in California 35 years ago, the Sunday Times of London reported.
Farrow, who starred in Polanski's 1968 film "Rosemary's Baby," has told friends she plans to go to London this month and testify on the director's behalf.
Polanski has told his lawyers that he was "utterly distraught" over the magazine's claim. He was abroad when his wife, Sharon Tate -- who was then eight months pregnant -- was stabbed to death by followers of Charles Manson in her home in Bel Air, Calif.
Polanski has been a fugitive from American justice since 1978, when he left the United States while on bail on a rape charge involving a 13-year-old girl. He has been allowed to give evidence in the libel trial by video link, because he has been told that he could be arrested and charged with rape if he came to Britain.