The arrest of four Trinity Mirror journalists was prompted by information from a tabloid supergrass, according to reports.
Tina Weaver, the former editor of the Sunday Mirror and deputy editor of the Daily Mirror under Piers Morgan, was arrested with Sunday People editor and deputy editor James Scott, plus Nick Buckley and Mark Thomas, former People editor and deputy editor of the Sunday Mirror respectively.
The four were arrested by detectives with Operation Weeting on suspicion of conspiracy to intercept voicemail messages. Following the controversy, Trinity Mirror's share price plunged to a six-week low, wiping £60m from the firm's value
The four were arrested after Metropolitan Police received fresh information from an "insider with knowledge of the workings of a number of tabloid titles" who had significant information about the Sunday Mirror and the News of the World, according to the Independent.
The paper claimed that Scotland Yard had received a number of emails from current and former Mirror Group executives which were used as evidence.
A Trinity Mirror spokesman said the company was cooperating with the police but denied that the company passed on emails to investigating officers. It is believed the insider emails were forwarded recently.
Former Trinity Mirror chief executive Sly Bailey gave evidence at the Leveson Inquiry into press standards in which she said the Mirror Group was a "healthy" company and there was no evidence of Mirror Group journalists breaking the law.
The Independent quoted sources saying that police had drawn up a list of possible victims whose voicemails may have been illegally accessed by Mirror Group journalist between 2003 and 2004.
These included former England football manager Sven-Goran Eriksson, former Coronation Street actress Shobna Gulati, and Abbie Gibson, the nanny of David and Victoria Beckham.
The arrests were part of separate arm of Operation Weeting, which has so far focused primarily on News International, the publisher of the now-defunct News of the World.
All four have been released on bail.