RBoS Action Group, which represents the largest group of shareholders, said it had filed claims by yesterday and would lodge more next week. The claims could be for at least £1.2bn.
That adds to claims from three other groups that total more than £2.5bn on behalf of big financial institutions and thousands of small investors in the first American-style class action set to hit English courts.
Last Thursday marked the six-year anniversary of when the shares RBS sold in its emergency rights issue began trading and could be the cut-off point under English law after which damages claims are no longer be accepted.
Fred Goodwin, the former boss of the now state-backed RBS, asked shareholders to provide £12bn at the height of the credit crisis to shore up the bank's capital position, which fell dangerously low after it paid a premium price for parts of Dutch peer ABN Amro and lost billions on US credit market assets.
Under Mr Goodwin, dubbed "Fred the Shred", RBS staged a meteoric rise to global prominence, expanding so aggressively into wholesale banking that its near-failure at the height of the credit crisis threatened to fell the UK financial system. Shareholders lost around 80 per cent of their investments.
"This is a novel and unique case. There has never been anything quite like it in the English courts before," said Clive Zietman, a partner at law firm Stewarts Law, which is representing 313 UK and international institutional clients including local authorities, pension funds and asset managers.
Mr Goodwin, alongside former chairman Tom McKillop and two other former executives, is likely to be called as a key witness if the case goes to trial, although sources said that may not happen until early 2016.
RBS rejects allegations its former directors misled investors or acted illegally.
"These things will be set out in court rather than in an early settlement, we have a good defence on this," RBS Chief Executive Ross McEwan said.
RBS is being run by a new team and is 80 per cent owned by the government, which was forced to step in with a £45bn taxpayer-funded bailout.
Investors claim RBS failed to disclose how bad its capital position was; that its risk management and controls were "fundamentally flawed"; and that the integration of the ABN Amro business was not going well.
Investors represented by Stewarts Law are claiming more than 1.3bn, while law firm Quinn Emanuel has filed claims on behalf of five of the biggest institutions - including top 10 investors at the time Legal & General, Standard Life and Prudential.
The main uncertainty about the size of claims RBS faces is from the RBoS Action Group. The group said it has signed up about 13,500 retail investors and about 100 institutions.