Bank Confidential, a new organisation which aims to root out unacceptable practices in the big banks, said it had become aware of allegedly fraudulent practices in the bank from whistleblowers.

One of the purported whistleblowers, who says he worked for RBS for more than 12 years, told The Mail on Sunday that he was taught how to copy customer signatures on his first week of training as a finance manager in 2005.

He said: “The manager told me I could download signatures which were stored on the signature verification section of a customer’s records, then trace it on to another document where the signature was missing.”

Liberal Democrat MP Norman Lamb said: “These are very serious allegations. I am trying to secure a face-to-face meeting with the FCA and its head, Andrew Bailey, to ensure these claims are investigated. I am also contacting police about the fraudulent behaviour, which is of course, a criminal matter.”

A spokesperson for RBS said: “RBS takes any allegations of misconduct very seriously. We have robust whistleblowing processes in place, as required by the regulator. Many of these allegations date back a number of years and have been thoroughly investigated by RBS and other bodies, including the courts.

“No evidence has been found to support them. We deny manipulating or falsifying customer records to suit our purposes.”