A former Sunday Mirror journalist has admitted hacking phones after voluntarily coming forward to confess to police.
Graham Johnson, 46, pleaded guilty at Westminster Magistrates' Court to intercepting communications in the course of transmission without lawful authority.
The court heard that Johnson, who worked at the Sunday Mirror between 1997 and 2005, hacked a phone to investigate whether a soap star was having an affair with a gangster in autumn 2001.
After journalists for Mirror Group Newspapers were arrested in March last year, Johnson came forward to confess to a "short and intense" period of hacking lasting three to seven days.
The court heard that he had been "shown by a senior person in a supervisory capacity how to access voicemails" and he was not aware that it was a crime at the time.
Prosecutor Luke Dockwray said: "I should make clear that he was not under investigation when he contacted police."
Johnson, from Greenwich in south east London, had been trying to find out whether the soap actress was meeting the gangland boss to buy drugs.
The court heard that he listened to between 10 and 13 messages a day for a period of up to seven days.
A story was subsequently published in the Sunday Mirror, with Johnson receiving the second byline.
Avtar Bhatoa, for the defence, said his client "discontinued the hacking because he did not feel it was right".
Bhatoa said: "I should point out that this inquiry was into what he thought to be a legitimate gangland story and that he was shown by a senior person in a supervisory capacity how to access voicemails.
"At the time he did not know that this was illegal but after hacking the phone for less than a week, he discontinued the hacking because he did not feel it was right.
"When, 12 years later, arrests were made, he straightaway contacted police the next day. He came forward and faces sentencing."
District Judge Quentin Purdy said he did not have sufficient powers to deal with the matter and Johnson would be sentenced at the Old Bailey on November 27.
The judge said: "Great credit comes your way for pleading guilty today and even more so for literally turning yourself in and voluntarily throwing yourself at the mercy of the system.
"However, it was a grave intrusion into other people's business. Indeed, the history of recent years has shown how serious this kind of intrusion can be.
"In my judgment, given the gravity of the intrusion, even during a relatively short period of time, this court does not have the powers to deal with the matter."
Johnson, who is originally from Liverpool, declined to comment as he left court, saying Bhatoa had advised him not to.
He was investigations editor at the Sunday Mirror for six years after previously working at the now-defunct tabloid the News of the World from 1995 to 1997.
Other publications which have published his work include The Observer, Vice, The Guardian and the Liverpool Echo.
There is no suggestion that he did anything illegal for those other publications.
In addition to his work as a print journalist, Johnson has published eight non-fiction books, two novels and has worked on several documentaries.