(Reuters) - An investigation into Rupert Murdoch's top-selling British newspaper, the Sun, has uncovered evidence that it paid tens of thousands of pounds in retainers to public officials for tipoffs, a source with knowledge of the probe said on Wednesday.

Much of the evidence passed to police has been provided by Murdoch's own News Corp group, and deepens a crisis at the Sun, where officers have arrested nine former and current senior staff in recent weeks over illegal payments.

Murdoch has been trying to regain the high ground ever since an outcry last summer - over revelations that his journalists had hacked the voicemails of crime victims and their families - forced him to close the profitable News of the World title and abort a planned multibillion-dollar buyout of Britain's biggest satellite broadcaster.

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