Channel 4 News has learned that RBS has been seizing properties from customers only for a subsidiary of the bank to buy them at knock-down prices.
Small businesses have told Channel 4 News that when their property assets were seized by RBS they were sold off at auction to another arm of the business called West Register.
The bank has sold properties belonging to debtors at below-market value after suddenly withdrawing credit.
Property developer Chris Kashourides had a building in north London sold off for just £415,000 when the bank unexpectedly gave him seven days to pay off his overdraft in 2010.
Four months later the property was resold on the open market for more than £1 million. Mr Kashourides was forced to sell 25 more of his properties at reduced prices thanks to pressure from the lender.
He blames swaps – a complex insurance product designed to protect borrowers from rising interest rates – for his predicament.
As interest rates tumbled during the financial crisis, the cost of having a swap soared, pushing many small businesses to the wall.
The Financial Conduct Authority is now investigating allegations of widespread mis-selling of swaps by the big banks, who have been accused of being slow to compensate firms who bought the product inappropriately.
But some companies go further than allegations of mis-selling in their criticism of RBS.
During the boom years RBS, more than any other bank, lent billions of pounds to the property sector. The financial crash came and the bank was desperate to reduce its exposure to property as values started to fall.
Now some small businesses claim the bank has deliberately pulled the plug on them to hasten its exit from the property market.
And the fact that as subsidiary of RBS bought some of the properties the bank was selling off demands an investigation, according to some industry campaigners.
Jeremy Roe from Bully Banks said: “It’s a scandal that needs to be investigated. What was a major high street bank doing acquiring such a substantial property portfolio from its customers?
“If you stand back and look at what RBS has done you have to say questions come immediately to mind and they are questions which we’re saying require investigation and require an answer.”