The publisher of the Sunday Mirror announced today that it was being investigated by Scotland Yard for alleged phone hacking by former employees.

Trinity Mirror said police had informed MGN Limited, its national newspaper publishing subsidiary, that an inquiry is under way to see if it is criminally liable for alleged unlawful conduct by ex employees at the weekly tabloid.

Several former Sunday Mirror journalists have already been arrested over hacking but this is the first news that the company itself is also being investigated.

Shares in Trinity Mirror dropped by more than 6 per cent soon after it announced the police investigation, although they later recovered.

The development means that the two of Britain’s biggest newspaper publishing groups are now thought to be under “corporate” investigation for phone hacking.

Last month it was reported that Rupert Murdoch's News International had been placed under investigation, though the Met has yet to officially confirm that claim.

The company, now rebranded News UK, has not commented on the claim.

Trinity Mirror said the investigation was at “a very early stage” and it said: “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."

Several former Trinity Mirror employees have been arrested in a separate strand of the phone hacking inquiry codenamed Operation Golding.

Former Sunday Mirror editor Tina Weaver, who worked at the paper between 2001 and 2012, was arrested in a dawn raid as part of the Metropolitan Police's Operation Weeting inquiry into phone hacking in March.

Ms Weaver, who was heavily pregnant at the time of her arrest, was held with former deputy editor of the newspaper Mark Thomas, current People editor James Scott and deputy editor Nick Buckley. All four were released on bail.

Trinity's Mirror's announcement comes after former Sunday Mirror and News of the World journalist Dan Evans was last week charged with phone-hacking offences.

He is accused of two counts of conspiring with others to intercept communications in their transmission, one of conspiracy to commit misconduct in public office and one of perverting the course of justice.

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.

During his inquiry into press standards, Lord Justice Leveson described former Daily Mirror editor Piers Morgan's claim that he had no knowledge of alleged phone hacking at the newspaper as 'utterly unpersuasive', and said the practice may well have occurred at the title in the late 1990s.

Mr Morgan, now a CNN chat-show host in America, left the paper over a fake picture scandal in 2004, has always denied any knowledge of hacking.

A Scotland Yard spokesman said he could not confirm Trinity Mirror's statement or whether News International, which has recently been rebranded as News UK, is under investigation as a corporate suspect.

He said: "As with any investigation we carry out, we do not identify suspects or anybody arrested or anybody we may we wish to speak to. That goes for corporations the same as it does for individuals."