A Guardian newspaper reporter has been questioned under caution by detectives investigating phone-hacking at the News of the World.Amelia Hill, who has broken major scoops about the scandal engulfing the Murdoch media empire, was quizzed by police probing alleged leaks from the investigation into News International.
It is understood the 37-year-old reporter had a friendship with a detective who works on Operation Weeting. A Scotland Yard officer has been arrested and suspended on suspicion of leaking information to the Guardian.
It marks an extraordinary twist in the five-year saga that has led to the arrest of 16 people, including Prime Minister David Cameron's spin doctor Andy Coulson, and the resignation of NI chief executive Rebekah Brooks.
A Guardian News & Media spokesman said: "We can confirm Amelia Hill has been questioned in connection with an investigation into alleged leaks. On a broader point, journalists would no doubt be concerned if the police sought to criminalise conversations between off-record sources and reporters."
In July Hill broke the Guardian's Milly Dowler story - described as a "tipping point" in the scandal - which revealed that private investigators working for the NoW hacked the voicemails of the murdered schoolgirl after she went missing.
Within weeks, Mrs Brooks, Metropolitan Police Commissioner Sir Paul Stephenson and Les Hinton, media mogul Rupert Murdoch's right-hand man for more than 50 years, had all resigned.
It is understood Hill received police tip-offs about impending arrests. The Evening Standard has been told the police officer suspended from Operation Weeting called the Guardian the night he was arrested asking to speak to Hill.
The Guardian has broken key scoops relating to the scandal which began when former NoW royal editor Clive Goodman was jailed for hacking the voicemails of royal aides in 2006.
In July 2009, the newspaper also revealed NI had paid £700,000 to football union boss Gordon Taylor in a bid to cover up alleged widespread phone-hacking at the now-defunct Sunday tabloid.
Dan Roberts, the Guardian's national news editor, said on Twitter that the developments were a "bleak day for journalism when (a) reporter behind vital hacking revelations is criminalised for doing her job".
Two separate internal inquiries into police relationships with the media were launched after it emerged that the NoW had allegedly paid more than £100,000 to corrupt royal protection officers in return for information.
An inquiry by Her Majesty's Inspectorate of Constabulary is examining "alleged corruption and abuse of power" in police relationships with the media. Separately, former Parliamentary Commissioner for Standards Elizabeth Filkin is drawing up a framework for how police officers handle their relationships with reporters.
Three years ago, a case against Sally Murrer, a reporter on the Milton Keynes Citizen, and a former Thames Valley police detective Mark Kearney was thrown out. Kearney had been accused of leaking information to her.
Meanwhile, police arrested another 35-year-old man at his home at 5.55am on suspicion of conspiracy to intercept voicemail messages. He was taken to a north London police station for questioning. He is the 16th suspect to be held since the scandal broke.
The dawn raid came as it emerged that Mr Coulson is refusing to give evidence to the House of Commons committee investigating the scandal.
MPs on the culture, media and sport select committee have asked him to comment on claims that he knew that hacking was widespread on the NoW when he was editor.