The investigation into payments by Rupert Murdoch's Sun journalists to police and other officials is looking into suspected criminality over a sustained period of time involving tens of thousands of pounds, a source with knowledge of the investigation told Reuters.

Police have arrested nine former and current senior Sun staff in recent weeks in an investigation looking into payments to police and public officials.

The move however has resulted in a backlash from staff who have accused police and Murdoch's News Corp management from conducting a witch hunt into journalists and their sources.

This is not about sources or expenses, this is an investigation into serious suspected criminality over a sustained period, the source said, speaking on the condition of anonymity.

It involves regular cash payments totalling tens of thousands of pounds a year for several years to public officials, some of whom were effectively on retainers to provide information. In totality it involves a six-figure sum.