A former Sunday Mirror journalist has pleaded guilty to intercepting voicemail messages in 2001.
The paper's former investigations editor, Graham Johnson, is the first Mirror Group Newspapers journalist to admit to phone hacking.
Westminster Magistrates' Court heard that Mr Johnson, who will be sentenced at the Old Bailey on 27 November, voluntarily contacted police in 2013.
He worked at the Sunday Mirror between 1997 and 2005.
A spokesman for Trinity Mirror said the company would not be making a comment on Mr Johnson's conviction.
Mr Johnson came forward in March 2013 after hacking a phone to investigate whether a soap star was having an affair with a gangster in autumn 2001.
The court heard that he had been "shown by a senior person in a supervisory capacity how to access voicemails" and that he was not aware that it was a crime at the time.
He listened to between 10 and 13 messages a day for a period of up to seven days, the court was told.
He confessed to a "short and intense" period of hacking lasting three to seven days.
Granting unconditional bail, District Judge Quentin Purdy praised the 46-year-old for admitting his actions.
He 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."
Mr Johnson previously worked at the now defunct News of the World from 1995 to 1997 and has had work published by others including the Guardian and BBC Panorama.
He has also written eight non-fiction books and two novels.
In September, Trinity Mirror admitted for the first time that some of its journalists were involved in phone hacking.
Former England manager Sven Goran Eriksson, entertainer Shane Richie and actor Christopher Eccleston are among those to have received compensation from the newspaper group.
Trinity Mirror publishes titles including the Daily Mirror, Sunday Mirror and Sunday People.