Around 17 million pounds was wiped off the value of Trinity Mirror after a phone hacking scandal spread to the owner of the British tabloid Daily Mirror.
On Monday the lawyer who handled many of those phone-hacking cases filed legal claims against Trinity Mirror on behalf of four people, including the former England soccer manager Sven-Goran Eriksson.
The reputation of British journalism was rocked by the revelation of hacking by journalists at Rupert Murdoch's now defunct News of the World into voicemails of celebrities, politicians and even crime victims.
Trinity Mirror said in a statement it had yet to receive any claims, and it repeated assurances that its journalists worked within the law.
"We have not yet received any claims nor have we been provided with any substantiation for those claims," Trinity Mirror said in a statement.
Shares in the group were down 9.4 percent at 0121 p.m. as investors digested the threat of increasing legal costs and damage to the reputation of a group that is already grappling with falling advertising and circulation revenues.
News International has settled dozens of civil cases, paying tens of millions of pounds, and its journalists are still facing criminal charges over the allegations that its staff routinely hacked into voicemail messages to generate salacious stories.
Billions of dollars were wiped off the value of Murdoch's News Corp at the height of the scandal last year and executives were forced to step down.
Singer analyst Johnathan Barrett said the legal claims would weigh on Trinity Mirror's share price because although the number of claims was small compared with News International, they could rise over time.
The legal action will also provide a stiff test to the group's new chief executive Simon Fox, who joined the publisher in September and to Piers Morgan, a former Daily Mirror editor who is now a CNN talk-show host in the United States.
The claims allege phone hacking took place at the Daily Mirror when Morgan, who previously edited Murdoch's News of the World, was editor.
Morgan has denied authorising phone hacking during his time as editor of the Mirror, most recently at the Leveson Inquiry, which was set up to investigate the conduct of the press. It is due to report in the coming weeks and is expected to heavily criticise the British media.
The other three claimants are Abbie Gibson, a former nanny for the family of soccer player David Beckham, soccer player Garry Flitcroft and actress Shobna Gulati.
(Additional reporting by Rhys Jones; Editing by Erica Billingham and Louise Heavens)