Police are widening an inquiry into allegations of phone-tapping of staff in Prince Charles's household.
Complaints from staff at Clarence House prompted the investigation
They will examine whether other public figures have had calls intercepted. Media reports suggest a Cabinet minister may have been spied upon.
Three men were arrested in south London on Tuesday as part of the inquiry.
The inquiry began after details of a meeting an ITV reporter was to have had with Prince William were published in the News of the World.
The newspaper's Clive Goodman was among those arrested.
The police inquiry, which began more than seven months ago, has now spread to include other public figures.
BBC News royal correspondent June Kelly said the wider investigation did not include the prime minister.
ITV News political editor Tom Bradby, who at the time was the network's royal correspondent, told the ITV Lunchtime News: "When he [Prince William] and I eventually hooked up we looked at each other and thought 'How on earth did that get out'.
"Then we started discussing one or two other things that had happened recently.
"There had been a meeting he had with a knee surgeon that again only he and his personal secretary and the knee surgeon had known about that had gotten into the News of the World.
"And basically the answer we came up with was that it must be something like breaking into mobile answer machines messages."
Security expert Mark Cooper told the BBC that accessing voicemail messages stored on an individual's mobile phone was "reasonably straightforward."
Mr Cooper said the indications so far were that voicemail messages left by members of the royal household had been listened to by third parties.
Complaints by staff at Clarence House, the official residence to the Prince of Wales, prompted the investigation and led to the men being arrested on Tuesday.
They were questioned at London's Charing Cross police station and a 50-year-old man, not Mr Goodman, has now been released on bail, police said.
Officers have not ruled out the possibility that other royal households, including Buckingham Palace, could have had their phones intercepted, or that politicians and other members of the Royal Family could have had their voicemail messages monitored.
Publicist Max Clifford said he had been contacted by his mobile phone provider after "irregularities" with his voicemail, but said it may just be "an amazing coincidence" and not connected to the royal investigation.
The chairman of the Press Complaints Commission, Sir Christopher Meyer, said he had heard rumours about journalists using interception techniques to obtain information.
"One hears stories and rumours all the time that this may be going on. Nobody has come to me with hard evidence of this," he told BBC Radio 4's Today programme.
Scotland Yard said the claims had "potential security implications" so the investigation is being handled by the anti-terrorist branch.
Scotland Yard said: "Police launched an investigation after concerns were reported to the Met's Royalty Protection Department by members of the Royal Household at Clarence House.
"It is focused on alleged repeated security breaches within telephone networks over a significant period of time and the potential impact this may have on protective security around a number of individuals."
Officers have been working with the telephone firms for four months and have uncovered mobile phone numbers that may belong to other members of the Royal Family, politicians and celebrities.