It is understood that the bank as well as its former board of directors, including former chairman Sir Tom McKillop and chief executive Fred Goodwin, will be named as defendants in the misrepresentation claim.
The dispute centres on RBS’s £12bn rights issue, which was launched in May 2008 just months before the bank was rescued from near-collapse by a Government bailout.
It is claimed that there were discrepancies in the rights issue prospectus, which allegedly led investors to believe that they would benefit from the capital raising.
The issue raised £12bn, but then the banks’ share price collapsed from 200p to 36.74p.
Marshall said the claim was in the preliminary stages with a writ due to be issued in the coming months.