The former insiders who have agreed to appear for the RBoS Shareholders' Action Group include one banker who was an RBS director for more than a decade. The case is expected to be heard in May.
Victor Hong, a former managing director of risk management and head of fixed-income independent price verification at US-based RBS Greenwich Capital Markets, is one of those who have agreed to give evidence against the bank. He resigned from RBS in October 2007 alleging that senior colleagues disregarded his advice to mark down valuations of the bank's collateralised debt obligations, the mortgage-backed securities that were central to the 2008 crash.
The investors group claims that RBS and four of its former directors - Fred Goodwin, Guy Whittaker, Johnny Cameron and Sir Tom McKillop - effectively duped them into pumping £12.3bn into its shares five months ahead of the bank's October 2008 collapse which resulted in its £45.5bn bailout, the world's largest. The bank sold investors the shares at £2 each in June 2008, but their value had fallen to just 10p seven months later.
It emerged last week that RBS - which has shed 40,000 staff since the crash - was preparing to cut thousands more jobs across the UK to reduce costs by around £1bn.
Other ex-RBS insiders who have agreed in recent weeks to be witnesses in the shareholder action include a former senior member of the team which oversaw the ill-fated integration of RBS and Dutch bank ABN Amro, and a former senior internal auditor, who reported directly to RBS's former finance director, Guy Whittaker, one of the defendants in the case.
Another new witness for shareholders is a former senior member of RBS's investor relations team, who claims to be aware of messages that RBS was privately conveying to large City investors at the time. And a former senior executive in RBS Global Banking and Markets, the institution's investment banking arm, earlier agreed to provide testimony about the circumstances surrounding the rights issue.
The group of 16,000 investors - which includes institutional and corporate investors as well as 12,000 individual investors - filed its original High Court writ in April 2013.
It alleges the bank's April 2008 share prospectus was defective and contained "material mis-statements and omissions". It also claims the document falsely portrayed RBS as financially strong, failed to admit to the inadequacies of its back office systems and controls, and failed to admit it had incomplete knowledge of its own financial position.
Legal sources suggest RBS may settle "by Christmas". One told the Sunday Herald: "My gut feel is that, if RBS were to offer investors £1.40 a share, they would accept it."
A spokesman for RBS said: "While RBS and its former directors made some business decisions that have been criticised, this does not mean that they misled investors or acted illegally. We believe we have strong defences to the claims that are being brought against the group and that is why we intend to defend these vigorously."