Sienna Miller accused her family and friends of selling stories to the media after journalists obtained intimate information about her by hacking her phone, the Leveson Inquiry heard today.
The actress described how she felt "terrible" for even considering that those closest to her could betray her in this way.
She told the inquiry into press standards that she changed her phone number three times in three months after becoming concerned that personal details were finding their way into newspaper stories.
Having switched her number repeatedly, Miller said she was "pretty convinced" the leaks could not be the result of phone hacking and so accused her close friends and family of being the source.
The actress, whose films include Layer Cake, Alfie and Stardust, said a reporter found out about a particular "very private" piece of information which she had only told to four people, including her mother.
"I am very lucky, I have a very tight group of friends and a very supportive family, and to this date no-one has ever sold a story on me," she said.
"But it was baffling how certain pieces of information kept coming out and the first initial steps I took were to change my mobile number.
"And then I changed it again and again, and I ended up changing it three times in three months."
She added: "Naturally, having changed my number and being pretty convinced that it couldn't be as a result of hacking, I accused my friends and family of selling stories and they accused each other as well."
Miller went on: "I feel terrible that I would even consider accusing people of betraying me like that, especially being people who I know would rather die than betray me.
"But it just seemed so entirely paranoid to assume that your house is being bugged or you're being listened to somehow."
Miller described becoming "constantly very scared and intensely paranoid" and feeling "very violated" by the media intrusion in her life - every area of which she claimed was "under surveillance".
Journalists and photographers would turn up at places where she had arranged to meet someone on the phone, the inquiry heard, "baffling" her that they knew where she was going before she had arrived.
"I felt like I was living in some sort of video game and people pre-empting every move I made, obviously as a result of accessing my private information," she said.
The inquiry also heard how a photograph of the actress was published in such a way as to suggest she was drunk when in fact she was playing with a child at a charity event.
"There was a very sick child that I was playing with in a corner of the room, who was pretending to shoot me and I was pretending to die," she said.
"The Mirror cut the boy out the photo and said I was drunk."
Miller sued the newspaper for this and a "minuscule" apology was printed, she said.
But for her it was too late, she suggested, as "the damage is done" and the image could have proved "detrimental to my career, to my reputation".
She added: "The fact they knew they would be sued and have to pay damages was really not enough of a deterrent."
The actress fought back against the media by obtaining a court order against the paparazzi in summer 2008 and also taking legal action against News Group Newspapers, the hearing was told.
Asked about her motive for the legal action against the News of the World publisher, she said: "It was not about financial compensation. I would rather have not gone through any of the litigation that I've had to go through."
But, she said, she "wanted to know who knew about this information, who had access to my phone numbers, who has been listening to me".
She is still waiting for full disclosure on this from News International, she added.
"So far it's been very unsatisfactory, what I've received," she said.
"I will continue to wait for it but its been a long process so far."
Miller said she was hounded by paparazzi for years until she obtained the injunction.
"I was relentlessly pursued by about 10 to 15 men almost daily, anything from being spat at or verbally abused," she said.
Photographers would drive dangerously to chase her, on one occasion nearly running over a pregnant woman, the inquiry heard.
The actress said: "I think there is something about the pursuit which is very exciting for paparazzi photographers."
Her lawyer Mark Thomson told the inquiry he advised her she had three choices to deal with this intrusion: ignore it, fight, or move to Paris - a reference to France's strong privacy laws.
Mr Thomson, whose other clients have included actor Hugh Grant and supermodel Naomi Campbell, said he believed that phone hacking extended beyond the News of the World and was "common industry practice".
Inquiry chairman Lord Justice Leveson asked him to provide a list of examples of the practice being used by other newspapers.
Mr Thomson was also critical of the newspaper and magazine regulatory body the Press Complaints Commission, which he said was not taken seriously by the press.
He told the hearing: "They (newspapers) don't want the PCC to be effective, in my view. They are quite happy with it as it is.
"They may say they want a few more tweaks to make it tougher, but as long as the PCC exists, this current activity will continue."
The lawyer raised the problem of the PCC's inability to regulate the paparazzi photographic agencies that supply celebrity pictures to the press.
He said: "The newspapers turn a blind eye to how they get their material as long as they have great photographs."
Mr Thomson also claimed that the press "invariably" denigrated people who took legal action to protect their privacy, citing the cases of Miller, Campbell and Canadian singer-songwriter Loreena McKennitt, who in 2006 won a Court of Appeal ruling upholding her right to privacy over a book by a former friend and employee.
He said: "My suspicion is (the newspapers decide), 'if they go to law, we will give them a good thrashing to deter other people from doing the same'."