By Paul Sandle

LONDON (Reuters) - British police are investigating whether Sunday Mirror newspaper publisher Trinity Mirror is criminally liable for alleged phone hacking by former journalists, widening a probe that had centered on Rupert Murdoch's News Corp.

Trinity Mirror said on Thursday it was being investigated by London's Metropolitan police and it was too soon to say how the matter might progress.

The practice of phone hacking by journalists to illegally obtain material for stories came to light two years ago at News Corp's now closed News of the World title.

Police arrested four former journalists from the Sunday Mirror tabloid in March - the first arrests related to a title outside Murdoch's stable - and said they were looking into a suspected conspiracy at Trinity Mirror's Mirror Group unit between 2003 and 2004 to hack into voicemails.

The editor of Trinity Mirror's other Sunday tabloid, the People, James Scott, and his deputy, Nick Buckley, were two of the people arrested.

Thursday's news widens the investigation from individual journalists to the company itself, following a similar pattern at Murdoch's British newspaper business, now renamed News UK.

"The group does not accept wrongdoing within its business and takes these allegations seriously," Trinity Mirror said in a statement.

Jonathan Coad, media lawyer at the firm Lewis Silkin, said that in certain circumstances companies could be charged with criminal offences based on offences committed by their employees in the course of their work.

"On basic legal principles, the police must firstly establish that the criminal activity was undertaken by Mirror employees during the course of their employment," he said.

"The second element they will have to establish is that senior management at Mirror Group Newspapers knew of and at least turned a blind eye to, or more particularly endorsed, this activity."

Shares in Trinity Mirror, which plunged to a four-month low of 81.5 pence in March following the arrests of individual journalists before gradually recovering, were down 2.5 percent at 125.4 pence by 0705 ET.

TRIAL

London police are already investigating Murdoch's News UK for possible criminal violations over allegations of phone-hacking and illegal payments to public officials by its journalists. News Corp has said it is aware of the police inquiry but has given no further details.

Dozens of current or former employees of the Murdoch newspapers have been arrested.

Among them are two former News of the World editors, Rebekah Brooks and Andy Coulson, who are due to go on trial on October 28 along with other defendants.

After leaving the News of the World, Coulson went on to work as Prime Minister David Cameron's spokesman, while Brooks, a friend of Cameron, rose to be chief executive of Murdoch's entire British newspaper business.

On Thursday, Britain's Crown Prosecution Service said Ben O'Driscoll, a former deputy news editor at News UK's Sun tabloid, had been charged with authorizing illegal payments worth 5,000 pounds ($7,900) to public officials for celebrity stories.

On September 3, former Sunday Mirror and News of the World journalist Dan Evans was charged with four offences in connection with phone hacking, including one in the period between 2003 and 2005.

He left the Sunday Mirror at the end of 2004.

($1 = 0.6324 British pounds)

(Additional reporting by Estelle Shirbon; Editing by Mark Potter)