Phone-hacking witness: arrest me if you want to quiz me as a suspect

A former News of the World journalist who came forward with information about illegal activities at the paper has challenged Scotland Yard to arrest him after detectives threatened to interview him as a suspect, not a witness.

Paul McMullan told the Guardian last week that the prime minister's media adviser, Andy Coulson, must have known his reporters were using illegal techniques when he was a senior editor at the News of the World. Coulson denies this.

McMullan was willing to give information to Scotland Yard, which has agreed to look at new evidence in the phone-hacking affair. However, when a senior detective approached him this week, he was told he would be interviewed "under caution", on the basis that anything he said could be used to prosecute him.

The same threat was made to Sean Hoare, the former News of the World reporter who told the New York Times that Coulson "actively encouraged" him to intercept voicemail messages. On Tuesday, Hoare was interviewed under caution and is understood to have told police that the use of the caution meant he was unable to help them.

McMullan, who has taken legal advice, is refusing to comply. He said: "In the past, I have been congratulated by the authorities for my undercover work exposing drug-smuggling, arms-trafficking and indeed corruption within the police. It is pathetic that they ask to interview me now under caution because the issue is politically topical. If they want to treat me as a suspect, they will have to arrest me."

McMullan was deputy features editor and then an investigative reporter during an 18-month period when Coulson was deputy editor of the News of the World. He told the Guardian he commissioned private investigators to commit several hundred acts which could be regarded as unlawful, that the use of illegal techniques was no secret at the paper and that Coulson must have been aware of them.

Coulson has denied knowing anything about any illegal activity by his journalists. He resigned on that basis in January 2007 after his royal correspondent, Clive Goodman, and the paper's contracted private investigator, Glenn Mulcaire, were jailed for illegally listening to the voicemail messages of eight public figures.

Since then Scotland Yard has been criticised for failing to investigate the evidence it collected during its inquiry. It has emerged that, apart from Goodman, police interviewed no other journalist or manager from the paper even though they seized paperwork which implicated others in handling intercepted voicemail. Following stories in the New York Times and the Guardian, police agreed to interview journalists who were named by newspapers as sources, but have chosen not to find any witnesses of their own.

MPs have expressed concern that the decision to interview Hoare and McMullan under caution might intimidate other potential witnesses who may have considered speaking out. David Winnick, a Labour member of the House of Commons home affairs select committee, said this week: "All this seems very strange. I can well understand that those who thought they could put their part of what happened, may now say to themselves they do not want to find themselves being questioned by police under caution."

Meanwhile, the attorney general, Dominic Grieve, has held a meeting with the director of public prosecutions, Keir Starmer, to discuss phone hacking.Westminster sources suggested the meeting was an indicator of the government's concern over the implications of the affair for Downing Street and for Scotland Yard. The DPP's office refused to comment.