Tina Weaver, the former editor of the Sunday Mirror and member of the Press Complaints Commission, is believed to be one of four high-profile arrests on phone hacking allegations.
Tina Weaver, editor of The Sunday Mirror, arrives to give evidence at The Leveson Inquiry
Heavily pregnant Weaver is believed to have been arrested at 6am this morning along with Sunday People editor, James Scott, who would be the first serving newspaper editor arrested over alleged phone hacking, in relation to his time at the Sunday Mirror.
The other two arrests are believed to be Nick Buckley, the deputy Sunday People editor, and Mark Thomas, the former People editor.
Weaver has been a household name in tabloid journalism for over twenty years, beginning her career on the Sunday People, later working on Today, and eventually Head of Features then Deputy Editor of the Daily Mirror.
In April 2001 she was appointed Editor of the Sunday Mirror, and in May 2012 she was made redundant by publisher Trinity Mirror.
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."
Thursday'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 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.