Publisher of Daily and Sunday Mirror says Met is investigating if it is 'criminally liable' for alleged hacking by previous employees

The publisher of the Sunday Mirror has said it is under investigation by the Metropolitan police over alleged phone hacking. Trinity Mirror, which also publishes the Daily Mirror, told the Stock Exchange the Met was investigating whether it was criminally liable for an alleged phone-hacking conspiracy by previous employees.

The announcement is the clearest admission by the newspaper group that it is under investigation as part of Operation Weeting, the Met's two-year inquiry into alleged phone-hacking.

The statement said: "Trinity Mirror plc notes that its subsidiary, MGN Limited, publisher of the group's national newspapers, has been notified by the Metropolitan police that they are at a very early stage in investigating whether MGN is criminally liable for the alleged unlawful conduct by previous employees in relation to phone hacking on the Sunday Mirror.

"The group does not accept wrongdoing within its business and takes these allegations seriously. It is too soon to know how these matters will progress and further updates will be made if there are any significant developments."

The Met's move is a serious development and indicates the force is investigating MGN for a corporate charge as well as individual journalists who have been arrested.

MGN is obliged to inform shareholders of any development that could have a material impact on its stock. Trinity Mirror shares fell 5.5% in early trading but rallied to end down 3% at 125p.

News International, now rebranded News UK, did not reveal it had been told by the police that it was being investigated for corporate charges in May 2012, possibly because the company was not separately listed on the London stock market. However it emerged in the Leveson inquiry in July 2012 when the head of the investigation into alleged phone hacking and unlawful payments for stories, Sue Akers, gave evidence.

It was reported last month that one of Rupert Murdoch's most senior lawyers had been interviewed under caution on behalf of the company and two other very senior figures have been officially cautioned for corporate offences.

It has also been widely reported that American authorities are investigating Murdoch's companies in the US for possible corporate charges for alleged violation of the Foreign Corrupt Practices Act.

This month Dan Evans, a former Sunday Mirror reporter, became the first journalist to be charged over an alleged conspiracy to hack phones who was not working at the News of the World. Evans, who worked at the Sunday Mirror between 2002 and 2004 and the News of the World from 2005, is accused of four counts of alleged criminal activity, including a conspiracy to hack the phones of "well-known people and their associates" between 2003 and 2010.

Scotland Yard first announced in March that it was investigating what it alleged was a separate conspiracy to the phone-hacking scandal at the News of the World.

On 14 March the Met arrested four former Sunday Mirror journalists – including its former editor, Tina Weaver – on suspicion of involvement in an alleged phone-hacking conspiracy in 2003-4.

The other three ex-Sunday Mirror journalists arrested were the Sunday People editor, James Scott, his deputy, Nick Buckley and Mark Thomas, the former People editor. All four were released on police bail.