Evidence from a "supergrass" is understood to have prompted yesterday'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 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.
Trinity Mirror shares plunged 20 per cent on the London stock exchange, wiping £60 million ($110 million) from the firm's value. Earlier in the day the company announced a 75 per cent fall in profits.