Four journalists were arrested today on suspicion of phone hacking at Mirror Group Newspapers, Scotland Yard said.
Three men, aged 40, 49 and 46, and a 47-year-old woman were arrested at separate addresses in south London at 6am this morning.
In a statement, the force said detectives working on Operation Weeting, Scotland Yard's investigation into phone hacking, "have identified and are investigating a suspected conspiracy to intercept telephone voicemails at Mirror Group Newspapers".
It added: "This conspiracy is being treated as a separate conspiracy to the two being investigated at the now defunct News of the World newspaper.
"It is believed it mainly concerned the Sunday Mirror newspaper and at this stage the primary focus is on the years 2003 and 2004."
Today's arrests fall under a separate alleged conspiracy to the two being investigated at the now defunct News of the World newspaper, Scotland Yard said.
The men and woman arrested are journalists or former journalists on Mirror Group Newspaper titles, it added.
The suspects are being interviewed at various police stations in London while searches are being carried out at a number of addresses.
Officers said they plan to make contact with people they believe to have been victims of the suspected phone hacking in due course.