Evidence from a “supergrass” is understood to have prompted today’s arrests of the former Sunday Mirror editor Tina Weaver and three other executives from the Mirror Group on suspicion of phone hacking.

An insider with knowledge of the workings of a number of tabloid titles is thought to have handed the Metropolitan Police significant new information about the Sunday Mirror and the News of the World.

Scotland Yard is also thought to have obtained evidence from a recent exchange of emails between a small group of current and former Mirror Group executives. Detectives have already drawn up a list of preliminary list of possible victims whose voicemails may have been illegally accessed by Mirror Group journalists. The disclosures have opened a new front in Scotland Yard’s inquiries into illegal news-gathering by tabloid journalists.

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Trinity Mirror shares plunged 20 per cent on the London stock exchange, wiping £60m from the firm’s value. Earlier in the day the company announced a 75 per cent fall in profits.

At 6am today, detectives from Operation Weeting raided the homes of Mrs Weaver, who edited the Sunday Mirror for 11 years, until she was ousted last year in a surprise move, and her former deputy (and former People editor) Mark Thomas.

James Scott, current editor of the Sunday Mirror’s sister paper the Sunday People, and his deputy Nick Buckley, both based in the paper’s offices in London’s Docklands, were also arrested. They are the most senior serving journalists to be arrested in the police inquiries into press misbehaviour, which have so far led to more than 100 arrests.

All four were taken to police stations in London and questioned about a new alleged conspiracy to trawl the voicemails of newsworthy individuals for stories.

The operation cast a further shadow over Trinity Mirror, which owns both the Sunday Mirror and the People along with the Daily Mirror and scores of regional newspapers.

Until today Britain’s second-biggest newspaper group had largely avoided being dragged into the hacking scandal which has trashed the reputation of Rupert Murdoch’s News International and cost its owners at least £250m.

Scotland Yard said it was in the process of identifying a new series of victims of hacking “mainly” by the Sunday Mirror during 2003 and 2004. Since the Leveson Inquiry started last year, Trinity Mirror has held two internal investigations into phone hacking. A review headed by Paul Vickers, the company secretary and a barrister, questioned senior journalists.

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The former chief executive, Sly Bailey, subsequently told Lord Justice Leveson that Mirror Group was a “healthy” company and there was no evidence any Mirror Group journalist had ever broken the law.

Simon Fox, who took over after the departure of Ms Bailey, was forced by shareholders to hold another probe when civil actions over alleged hacking were lodged last October. Again all Mirror journalists were described as working within the law.

But The Independent understands that the company was told last year that police investigating possible hacking by Mirror Group journalists were concentrating on obtaining an “insider” account, and that arrests could be imminent.

The arrests were approved after an apparent supergrass familiar with both the News of the World and Mirror Group titles began providing information to police.

Officers are also examining a sequence of emails between key former and current Mirror Group personnel. It is believed the emails were sent recently. A Mirror Group spokesmen said the company was “co-operating with the police”, but it is understood that no emails have been handed to police by the firm.

Last year The Independent revealed details of evidence held by Scotland Yard which identified a former Mirror Group executive as paying a private investigations firm up to £125 a time for mobile phone numbers and pin access codes two years before phone hacking became a routine practice at the News of the World.

Police sources confirmed that Sven-Goran Eriksson, the former England football manager, Shobna Gulati, the former Coronation Street actress, Abbie Gibson, the nanny of David and Victoria Beckham, and the former captain of Blackburn Rovers football club had been identified in the preliminary list of potential victims of Mirror Group hacking. Claims for invasion of privacy against Trinity Mirror by Mr Eriksson and the three others were lodged in the High Court last year. Trinity Mirror has applied to strike out the claims.

Last month police arrested six journalists from the features department of the News of the World over “a further conspiracy” to hack phones.

Explaining the arrests, Scotland Yard said: “Detectives on Operation Weeting have identified and are investigating a suspected conspiracy to intercept telephone voicemails at Mirror Group Newspapers. This conspiracy is being treated as a separate conspiracy to the two being investigated at the News of the World.”