Actor poised to take legal action while it emerges Sean Hoare has been interviewed by police under caution

Sienna Miller was today expected to become the latest public figure to join the legal assault on Scotland Yard and the News of the World over the phone-hacking scandal.

Lawyers for the actor, whose private life has long been a tabloid staple, said she was ready to join the former deputy prime minister John Prescott and others in asking the courts for a judicial review of the police's conduct of the case.

Sources at the News of the World said she had taken the first step towards suing the paper for invading her privacy by allegedly listening to her voicemail messages.

The legal moves came after Miller, 28, discovered that the paper's private investigator, Glenn Mulcaire, had procured her mobile phone numbers, account number and secret PIN code, needed to intercept the voicemail of the minority of people who change the factory settings on their phones.

It also emerged today that Sean Hoare, a former News of the World journalist who told the New York Times and the BBC about hacking at the paper under Coulson, has been interviewed by police under caution. Lawyers said this indicated he was being treated by police as a suspect. "An interview under caution would follow someone being arrested as a suspect, except in the most exceptional circumstances," said Peter Lodder QC, a criminal barrister.

Paul McMullan, a second former News of the World staffer who told the Guardian that hacking was rife at the paper, was also contacted by detectives and told that they would like to question him under caution.

Scotland Yard has had the information about Miller being targeted in its possession since August 2006, when Mulcaire was arrested and his records seized, but failed to say anything to Miller until her lawyer, Mark Thomson, of Atkins Thomson, wrote to ask them. Even then an answer was delayed for 14 weeks.

Thomson said: "Our client only discovered that the Metropolitan police held this information on 15 October 2009 in response to our letter of 10 July 2009."Miller had previously sued the News of the World for invading her privacy by running intrusive stories about her relationships and publishing photographs of her in a bikini in Italy and of her filming a nude scene on a closed set.

In 2008, the paper settled the claims by paying her damages and sending her a written acknowledgement that they had been wrong to publish stories and photographs about her private life.

Friends of Miller said she had noticed warning signs with her voicemail, including new messages being recorded as old before she had listened to them.

For this reason, they said, she had changed her mobile number, but the material seized from Mulcaire suggested he had been able to discover her new number.

Former staff at the News of the World say Mulcaire was able to obtain confidential information from mobile phone companies and British Telecom.

Miller plans to join the legal action being run by Bindmans law firm to seek a judicial review of Scotland Yard's failure to warn potential victims of the hacking.

Those already involved in the action include Prescott, Brian Paddick, a former deputy assistant commissioner at Scotland Yard, the Labour frontbench spokesman Chris Bryant and the investigative journalist Brendan Montague.

Meanwhile it was revealed last night that MPs are to investigate whether parliamentary privileges were breached by Mulcaire. The Commons standards and privileges committee, which has been asked by MPs to investigate the matter, has decided to ask the clerk of the House and outside legal experts to advise on whether phone hacking is a contempt of parliament.

Miller is waiting for Scotland Yard to reply to a letter asking for an explanation of its failure to warn her, and Bindmans said she would join the legal action if a satisfactory response was not forthcoming.

During the original investigation in 2006, police told the director of public prosecutions that they would contact all "potential victims". But they failed to inform all those where there was clear evidence of successful hacking until after the Guardian revived the story last year and still have not warned all the owners of the 91 PIN codes found in Mulcaire's possession – Miller was one of them. Police continue to refuse to say how many people they have warned on the grounds that releasing the numbers would risk identifying the individualvictims.