LONDON (AFP) - British detectives arrested four journalists on Thursday over a suspected phone-hacking conspiracy at Mirror Group newspapers, widening the scandal that started at Rupert Murdoch's rival empire.

Scotland Yard said a new investigation focused on the Sunday Mirror newspaper, the main competitor to Murdoch's now defunct News of the World tabloid, in 2003 and 2004.

Three men aged 40, 46 and 49 and a woman aged 47, all current or former journalists, were arrested in dawn raids at addresses in London, the force said in a statement.

It said detectives on Operation Weeting, the probe into hacking at Murdoch's newspapers, "have identified and are investigating a suspected conspiracy to intercept telephone voicemails at Mirror Group Newspapers."

Police said the alleged crimes were "a separate conspiracy" to the investigations into phone-hacking and the bribery of public officials at News International, the British newspaper arm of Murdoch's News Corp. empire.

"It is believed it mainly concerned the Sunday Mirror newspaper and at this stage the primary focus is on the years 2003 and 2004," Scotland Yard said.

Around 100 people have been arrested under various phone-hacking and corruption investigations sparked by the scandal at the News of the World, which closed in disgrace in July 2010.

Several people have been charged including Prime Minister David Cameron's former media chief Andy Coulson and former News International chief Rebekah Brooks.