London police investigating the scandal of illegal telephone hacking centred on the British arm of Rupert Murdoch's News Corp organisation made another arrest today.

Police said a 31-year-old woman had been detained on suspicion of conspiracy to intercept voicemail messages and was being questioned in northeast England.

Detectives launched an investigation in January into whether journalists and private investigators, seeking gossip for stories, illegally intercepted voicemail messages on mobile phones of people ranging from celebrities and politicians to murder victims and the families of dead soldiers.

They are also looking at whether reporters paid police for information.

The probe is focused on the News of the World, which was closed down by News Corp's British arm News International in July after revelations its reporters had hacked the phone of schoolgirl Milly Dowler who was later found dead.

The woman is the 18th person to be arrested, joining a list of suspects which includes senior News International figures and Prime Minister David Cameron's former media chief, Andy Coulson, an ex-editor of News of the World.

The scandal has embarrassed and damaged Mr Murdoch and his son James, News International's chairman, and engulfed much of the British establishment, including senior police officers and Mr Cameron himself.

It also led to Mr Cameron ordering an inquiry into the British press, which began hearings this month, delivering more damaging allegations about the behaviour of newspapers, and suggestions editors were fully aware of the criminal activities.

Alastair Campbell today told the Leveson Inquiry the press had become "barely worth defending" and "quite frankly putrid in many of its elements".

In a written statement, Mr Campbell said he had been threatened after doing TV interviews in 2009 in which he talked about "systematic criminal activity" at the News of the World.

"I received a series of what can only be termed mildly threatening text and phone messages from senior journalists and executives at News International," he wrote.

Mr Campbell, a former political editor at the Daily Mirror newspaper and a suspected hacking victim himself, said he could not rule out that illegal methods went beyond the News of the World to other newspapers, although he said he had no evidence of this.

"I have also never understood how the Daily Mirror learned of Cherie's (Tony Blair's wife) pregnancy," he said in his written statement.

"As I recall it, at the time only a tiny number of people in Downing Street knew that she was pregnant."