According to a report in the Financial Times, Paul Rowley, a Lancashire hotelier, and his business partner John Green, were sold an interest rate swap in 2005.
After losing at the trial court, they are due to argue their case for redress to the Court of Appeal in mid-October.
The FCA has written to the court asking permission to participate. It had not taken up the cause of either side, but wanted to explain its rules and its interpretation of them to the court, the regulator said.
RBS won the initial case by arguing it had properly explained the risks.
UK regulators in January ordered UK banks to review thousands of interest rate swap sales, with a view to compensating any who were misled, which could lead to claims costing the industry £2bn, the FT reports.
