Review of local and natonal titles inlcuding the Daily Mirror is expected to last six weeks
The review will look at the publisher's regional newspapers, which include the Manchester Evening News, Liverpool Daily Post and Newcastle Evening Chronicle, and national titles the Daily Mirror, Sunday Mirror and the People.
It will be the first review of its kind at Trinity Mirror since 2004, when editorial controls and procedures were examined following criticism levelled at the BBC in the wake of the Hutton inquiry. It is expected to last six weeks, with the group's legal director Paul Vickers reporting back in mid-September.
A source at the company stressed that the review was a response to a general concern about practices within the media industry and not a specific investigation, but it comes at a time when the company is facing its own allegations of phone hacking.
In last week's select committee hearing on phone hacking, Conservative MP Louise Mensch claimed that former Daily Mirror editor Piers Morgan had admitted that hacking was used on an award-winning story uncovering an affair between Sven Goran Eriksson and Ulrika Johnsson in 2003.
Morgan denied Mensch's claim, and challenged her to repeat it outside of the parliamentary privilege which covers her committee comments. She refused to do so.
The Daily Mirror has also been hit by allegations from a former employee, James Hipwell, who worked under Morgan from 1998 to 2000. Hipwell told the Independent that phone hacking was "endemic" at the title and was "seen as a bit of a wheeze".
"You know what people around you are doing", he said.
"They would call a celebrity with one phone and when it was answered they would then hang up. By that stage the other phone would be into their [the celebrity's] voicemail and they would key in the code, 9999 or 0000. I saw that a lot."
Hipwell also said that he would be willing to give evidence to the public inquiry launched two weeks ago by David Cameron and led by Lord Leveson. The inquiry will look at phone hacking, corruption and media regulation.
Hipwell was fired by the Mirror in 2000 over the "city slickers" scandal, which saw him accused of buying shares and tipping them in the paper. He was sentenced to 59 days in prison.
A former Sunday Mirror journalist has also alleged that phone hacking took place at the Sunday edition. In an interview with BBC Newsnight, the former reporter, who was not named, claimed the practice was "quite widespread".
Trinity Mirror, which saw its share price fall almost 10 per cent in the wake of the allegations made against it, has issued a statement saying the claims are "totally unsubstantiated".
"Our position is clear. Our journalists work within the criminal law and the Press Complaints Commission code of conduct," a spokesman for the publisher said.
Trinity Mirror titles featured prominently in a 2003 investigation by the Information Commissioner's Office, Operation Motorman, which uncovered widespread use of private investigators by national newspapers and magazines.
Files taken from the office of investigator Steve Whittamore showed around 4000 requests from 31 news outlets, including 681 from the Daily Mirror, 143 from the Sunday Mirror and 802 from the Sunday People.
There were 952 requests from the Daily Mail, 266 from the Mail on Sunday and 228 from the News of the World.
Those requests do not necessarily relate to illegal behaviour, such as phone hacking or blagging.
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