The Scotland Yard investigation into alleged illegal payments by Sun journalists to police and other public officials is looking into claims that some individuals received more than £10,000 a year and were "effectively on retainer".

News Corporation's controversial internal unit passing information about alleged illegal practices by News International journalists to the Metropolitan police believes it has uncovered evidence of "serious suspected criminality over a sustained period" by some public officials supplying information to the Sun.

The revelation will almost certainly mean that Rupert Murdoch, the New York-based News Corp chairman and chief executive, will not seek to rein in the management and standards committee when he arrives in London on Friday for what are now almost certain to be crisis talks with News International management.

A source familiar with the operations of the MSC said the evidence uncovered after months of trawling through 300m internal News International emails and other documentation involved "regular cash payments totalling tens of thousands of pounds a year for several years to public officials, some of whom were effectively on retainer to provide information" to the Sun. "In totality it involves a six-figure sum," the insider added.

The source said that the investigation is not to do with "sources or expenses" claims by journalists.

Fresh details about the activities of the MSC, which is passing information on to the Met's Operation Elveden investigation into alleged illegal payments to public officials, come as the Sun's opposition to the unit grows, with the revelation on Wednesday that journalists from the paper have approached the National Union of Journalists about launching a legal challenge.

Information supplied by the MSC to the Met has led to the arrest and bailing of nine current and former Sun journalists, two police officers, an MoD employee and a member of the armed forces in relation to alleged illegal payments to public officials in the past three weeks.

The latest arrests of five senior Sun journalists on Saturday has led to near civil war at News International's Wapping HQ, with editorial staff accusing the MSC of throwing them "to the wolves". Journalists at News International say they feel "angry and betrayed".

The MSC was set up in July and operates independently of the Sun and other News International titles, reporting directly to News Corp in New York. But it has been on the back foot in the past week with barely concealed anger across all three Murdoch titles. The Times today weighed in with a comment piece attacking the MSC by leading human rights lawyer Geoffrey Robertson.

Scotland Yard has also extended its inquiry into alleged illegal payments by News International journalists to include all public servants.

The Met said it "recently uncovered evidence of suspected corruption by public officials who are not former or serving police officers" and has therefore "expanded its terms of reference".

In a statement issued on Wednesday it also confirmed that Operation Elveden was confined to News International titles.

"The public expects the police and public officials to behave in a certain way – professionally, ethically and with the utmost integrity – and the MPS has a duty to investigate evidence that indicates suspected criminal behaviour," the Met said.

"The unauthorised disclosure of information in return for payment is illegal and will be robustly investigated. That includes those who seek to corrupt or act as a conduit to aid corruption, which is also illegal."

Scotland Yard added that Operation Elveden is an "incredibly complex inquiry" and it is "of significant public interest" that anyone identified as being involved in wrongdoing would "receive the full weight of appropriate action through the criminal courts and/or disciplinary action".

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