By Tim Castle
LONDON (Reuters) - Police arrested four former editors from the Sunday Mirror tabloid on Thursday - the first journalists from a newspaper outside Rupert Murdoch's News Corp to be detained in a phone hacking scandal that shocked Britain.
Shares in Trinity Mirror fell 21 percent, wiping over 60 million pounds ($90 million) off its value after police said they were looking into a suspected conspiracy at its Mirror Group between 2003 and 2004 to intercept telephone voicemails.
Revelations about journalists hacking mobile phones - not only those of celebrities and politicians, but also crime victims, including a murdered schoolgirl - caused public outrage that led to the closure in July 2011 of Murdoch's News of the World - Britain's largest circulation paper at the time.
A public inquiry into press standards led to calls for better regulation of the news media and embarrassed Britain's political elite by revealing close relations between ruling politicians and newspaper editors and owners.
The issue is still a potentially dangerous one for Prime Minister David Cameron who had close ties to editors at the News of the World.
On Thursday he abruptly ended cross-party talks on press oversight and tabled a vote on light-touch rules, rather than the stronger legislation which some experts say is needed to curb media abuse - prompting allegations he is in thrall to the press barons.
Officers from the police's hacking inquiry - known as Operation Weeting - arrested the Mirror Group journalists at their London homes on suspicion of conspiracy to intercept telephone communications.
Mirror Group Newspapers includes three national titles: the Daily Mirror, the Sunday Mirror and the Sunday People.
Those arrested on Thursday were: the People's current editor James Scott and his deputy Nick Buckley, former Sunday Mirror editor Tina Weaver and former People editor Mark Thomas, Trinity Mirror Chief Executive Simon Fox told staff in an email, a source told Reuters.
Scott and Thomas are former deputy editors of the Sunday Mirror and Buckley was previously the paper's head of news. Weaver left the company last May when the group merged its Daily Mirror and Sunday Mirror tabloids into a 7-day operation.
"We take any allegation against employees very seriously and are co-operating with the police on this matter," Trinity Mirror said in a statement.
Trinity Mirror has previously said there was no evidence its journalists had hacked any phones.
But the group's shares lost around 17 million pounds in value in October when a lawyer who handled many of the original phone-hacking cases filed legal claims against Trinity Mirror on behalf of four people, including the former England football manager Sven-Goran Eriksson.
Tens of people from Murdoch's British tabloids have been arrested for hacking voice messages and for conspiring to make payments to public officials.
(Additional reporting by Kate Holton; Editing by Robin Pomeroy)