The new rules from the Financial Conduct Authority (FCA) ban credit brokers from charging fees and requesting payment details unless customers have clear information about who they are dealing with and the fees payable.
RBS, which also owns the NatWest and Ulster Bank brands, said recently it received as many as 640 calls a day from vulnerable consumers about attempts to charge fees on their accounts.
In one of the worst cases seen by RBS, someone looking for a £100 loan ended up being charged £700 because their details were passed around 10 different middlemen.
The rules come into force on January 2 after being fast-tracked by the FCA.
Chief executive Martin Wheatley said: “The fact that we have had to take these measures does not paint this market in a particularly good light.
“I hope that other firms will take note that, where we see evidence of customers being treated in a blatantly unfair way, we will move quickly to protect consumers from further harm.”
The FCA said consumers often do not realise they are dealing with a broker rather than a lender, with fees being taken without informed consent and under terms and conditions that are hidden or misleading.
It found that consumers are often misled as to the purpose of giving their payment details, while firms are passing on such information, without consumer consent, to other firms who also take a fee.
The FCA said that more than 40 per cent of the consumer credit complaints it receives relate to credit brokers - 80 per cent of which are to do with firms who charge upfront fees.
The watchdog is investigating a number of credit-broking firms and has stopped seven firms from taking on new business and, to date, three further cases have been referred for enforcement action.
Under today’s rules, firms will have to include their legal name, not just their trading name, in all advertising and other communications with customers.
They will also have to state prominently in all advertising that they are a credit broker and not a lender.
Consumers will also have a 14-day right of cancellation where credit-broking contracts are entered into as distance contracts, for example online.