Solicitors acting for Allan Clark and Gregor Cameron, former co-directors of Kilmarnock-based AJ Clark Construction Ltd and three associated companies, have written to the bank accusing it of abusing its dominant position to boost its own profits at their expense.
Lawyers for the pair have also written to the Office of Fair Trading (OFT), asking it to undertake an investigation into practices used by the bank's controversial Global Restructuring Group (GRG) against the companies.
The directors claim the bank removed hundreds of thousands of pounds from their account without notice, forced them to pay for costly external accountants, raised their borrowing costs without consulting directors and used a security prohibited by Enterprise Finance Guarantee rules.
Three of the four companies were placed in receivership and a fourth in administration in March, last year.
The directors claim that while the firms were faced with challenging economic conditions affecting the entire construction industry, they could have survived had the bank not placed it under "financial duress".
The directors' solicitor John Carruthers said he is waiting for the OFT to respond before moving to the next stage of legal proceedings.
An RBS spokesman said: "We are aware of the allegations made in this case and have found no evidence to support them."