The bank, 78 percent-owned by the British government, has hired the advisers to help its own probe which is running alongside an inquiry into the same matter by Britain's financial regulator, the sources said, declining to identify the advisers.

The Financial Conduct Authority (FCA) launched an investigation last year into allegations RBS pushed small firms into its turnaround division, called the Global Restructuring Group (GRG), so it could charge higher fees and interest and take control of their assets.

Lawrence Tomlinson, who acted as an adviser to former Business Secretary Vince Cable, had accused RBS of systematically sucking cash out of viable small businesses.

The allegations damaged RBS's reputation, undermining Chief Executive Ross McEwan's efforts to rebuild trust in the state-backed lender which was bailed out with 45.8 billion pounds of taxpayers' money during the 2007-09 financial crisis.

Senior RBS executives say that the allegations against GRG have been more damaging to the bank than other investigations into past misconduct including the rigging of benchmark interest and foreign exchange rates.

They are keen to draw a line under the issue as soon as possible with the government committed to selling at least three-quarters of its stake in the bank over the next five years.

"The one that is most reputationally damaging for us is the GRG case, suggesting we have acted badly towards our SME (small-and-medium-enterprise) customers," a senior source at the bank told Reuters this month.

The FCA launched a review into the turnaround unit last year, hiring consultancy Promontory Financial Group and Mazars, an accounting firm, to conduct it.

A separate review by law firm Clifford Chance, which was commissioned by RBS, found no evidence the bank set out to defraud small businesses.

The Clifford Chance report looked specifically into Tomkinson's allegation that GRG systematically sucked cash out of viable small businesses whereas the FCA is looking more generally at how RBS treated small firms in difficulty.

A source familiar with the matter said RBS's board decided to enlist outside help to ensure its responses to the FCA were not influenced by concerns about the bank's reputation and to identify any inconsistencies with the FCA's findings.

"It's to provide the board with confidence RBS is getting to the bottom of this," the source said.

(Reporting by Matt Scuffham, Editing by Sinead Cruise and Elaine Hardcastle)

By Matt Scuffham