RBS has so far estimated that it will have to pay out only £50m. But analysts at investment bank Nomura say their £850m estimate is based on RBS's small business exposure and the calculations of rival Barclays, which has a similar customer base and has already made a provision of £850m.
The two banks along with Lloyds and HSBC have just begun a review of all sales of interest rate swap agreements (IRSAs) and other hedging products, after a critical pilot study by the Financial Services Authority found mis-selling in 90% of past sales.
"RBS has 1.2 million SME customers, of which 5000 have live transactions," Nomura said.
Although RBS has said it sold fewer than 600 of the more complex IRSAs triggering automatic redress, that is still more than twice the combined sales of those products admitted by Lloyds and HSBC.
Nomura estimates that Lloyds will end up making a provision of £600m this year and HSBC £500m, making a total £2.8bn for the four banks, almost four times their original combined provisions of £750m.
Sir Philip Hampton, RBS chairman, told the annual meeting in Edinburgh last May that RBS had sold "simple risk management products" and that out of 67 complaints to the ombudsman only one had been upheld.
In August the bank made the £50m provision against mis-selling costs.