Mr McEwan said the bank was co-operating with regulators at the Financial Conduct Authority (FCA) to set up the probe, although he said it had received no evidence to support such a claim – but that it had done “serious damage” to RBS’s reputation.
The review, to be carried out by law firm Clifford Chance, will report by 31 January – with findings on specific allegations, as well as broader issues such as the banking culture.
RBS, which is 80 per cent owned by the taxpayer, said it would co-operate with any investigation by the FCA or the Bank of England’s Prudential Regulatory Authority.
The announcement came a day after Bank of England governor Mark Carney told MPs the allegations were “deeply troubling and extremely serious” and ought to be pursued “to the fullest extent of the law”.
Mr McEwan said: “Serious allegations have been made about how some of these customers were treated by the bank. We have to investigate those fully and we will. The review I have commissioned will examine the details of the allegations that have been made and will have access to whatever information it needs.
“It is important to note that the most serious allegation that has been made is that RBS conducted a ‘systematic’ effort to profit on the back of our customers when they were in financial distress.
“We do not believe that this is the case, but it has nonetheless done serious damage to RBS’s reputation. No evidence has been provided for that allegation to the bank. The review will investigate the claim fully and I will report back on its findings.”
Mr McEwan, who took over the top job last month, said it was vital that the bank enjoyed the trust of its customers. “I cannot allow these allegations to undermine that trust,” he said.
Mr Chance will conduct the review on an independent basis. It will be led by a regulatory partner within the firm and with a team that has had no previous dealings with the bank’s global restructuring group that is at the centre of the allegations.
The bank has started an internal inquiry into its treatment of small and medium-sized businesses (SMEs) following the damaging allegations in a report published on Monday by Lawrence Tomlinson, “entrepreneur-in-residence” at the Department of Business, Innovation and Skills.
However, banking industry critics of Mr Tomlinson’s report claimed it relies heavily on anecdotal evidence.
One banking industry executive at one of RBS’s rivals told The Scotsman: “It is just a nebulous catalogue of grievances, with no smoking gun at RBS of putting small businesses in harm’s way via RBS’s Global Restructuring Group.”
Yesterday’s developments came as the Serious Fraud Office (SFO) declined to confirm or deny reports that it is considering a criminal investigation into the allegations that RBS systematically defrauded companies by forcing them out of business.
An SFO spokesman said the body was “aware of the issue and monitoring developments”.
If the SFO does pursue a criminal investigation its ultimate sanctions include charging individuals or a corporate conviction.