The RBS department set up to help companies in trouble mistreated many of its clients, a leaked report for the Financial Conduct Authority says.
It found some "inappropriate action" - such as interest charges being raised or unnecessary fees added - was experienced by 92% of viable firms seen by RBS's Global Restructuring Group.
GRG operated from 2005 to 2013 and at its peak handled 16,000 companies.
The banking giant denies systematic abuse of its customers.
GRG would step in when businesses had skipped a loan repayment, or seen their sales and profits dip notably and was marketed as an expert service that would turn around a business.
But the report, seen by the BBC, commissioned for the City watchdog, the FCA, found struggling companies that were placed in the recovery group had a slim chance of emerging from it.
It said just 10% returned intact to the main RBS bank.
As of the end of 2014, 69% of firms, were still in the successor to GRG, which was supposed to return them to health.
Many of those businesses remained tied into complex loans with the bank in the form of derivatives linked to interest rates, from which it is often too expensive to leave.
Others ended up in administration, liquidation or a trade sale. According to one BBC source, instead of getting firms back on their feet, GRG was more like their "undertaker".
The 361 page leaked report also says the bank provided only "narrow compliance" to investigators.
Sources told the BBC investigators would regularly ask for details from the bank on certain matters and RBS would only provide the absolute minimum information.
RBS disputes this though. It says it provided investigators with "circa 323 gigabytes of data, comprising more than 15 million physical pages and 270,000 emails."