Sienna Miller believed a series of articles about her private life which appeared in the News of the World were leaked to the tabloid by a close friend or family member.
The actress, who was a victim of phone hacking, has said she was left so paranoid by stories published in 2005 and 2006, she accused her mum, sister and former boyfriend Jude Law of selling them to the press.
Sienna, who reached a £100,000 settlement with the paper and is assisting with the Leveson inquiry, also said she planted false stories after becoming suspicious her phone was being interfered with.
She told The Independent: "I changed my mobile number three times in three months.
"There were clicks on the line.
"I would pick up the phone and it would drop, there were messages I would never get, coupled with articles (containing private information) coming out every week.
"So I started to do tests. I would leave messages on people's phones, like we're going to rent this house or whatever, and it would appear next day in the papers."
After a number of stories appeared that only her inner circle could possibly know about, the actress, 29, gathered them together for a showdown.
"I sat down in a room with my mother, my best friend, my sister, my boyfriend, and said someone in this room is lying and selling stories and one of you has got to admit it."
Sienna - targeted because of her relationship with Jude - said she was still reluctant to take on Rupert Murdoch's media empire when the hacking scandal emerged for fear that it could damage her career.
Photo: ę Rex