Yorkshire Building Society and Credit Suisse have been fined a total of £3.8 million for misleading inexperienced customers over investments that had almost zero chance of achieving maximum returns.
The fines centred on a product called Cliquet designed by Credit Suisse International (CSI) and sold to nearly 84,000 customers who ploughed in £797 million.
It was aimed at "unsophisticated investors with limited investment experience" through distributors such as Yorkshire Building Society (YBS). YBS was responsible for three-quarters of the total invested, the Financial Conduct Authority (FCA) said.
The product offered a guaranteed minimum return plus the apparent potential for significantly more if the FTSE 100 performed well.
Regulators said the probability of achieving only the minimum return was 40-50% but there was almost no chance of the maximum return being achieved.
The FCA said the maximum return figure was given "undue prominence" in both CSI's product brochures, which YBS approved and provided to clients, and in YBS's own financial promotions.
YBS was fined £1.4 million and CSI £2.4 million. The building society saw 56,000 customers invest nearly £546 million. Almost 90% were aged 45 and over, with nearly a third of investors over 65.
Tracey McDermott, the FCA's director of enforcement and financial crime, said: "Financial promotions are often the primary source of information for consumers and in this case CSI and YBS let their customers down badly.
"These promotions were a serious breach of the requirement to be clear, fair and not misleading.
"CSI and YBS knew that the chances of receiving the maximum return were close to zero but they nevertheless highlighted this as a key promotional feature of the product. This was unacceptable."
The FCA said that in September 2010, following concerns raised by third parties including Which?, YBS had changed its promotions so that undue prominence was no longer given to the maximum return.
But it said that the lender continued to cite the potential for the return, giving an unfair impression of the likelihood of achieving it, the regulator said.
CSI also reviewed its promotions but decided not to make significant changes to its product brochure.
The FCA said both firms would have faced higher fines but agreed to settle at an early stage of the FCA's investigation.
They have each agreed to contact customers who bought the Cliquet product available between November 1 2009 and June 17 2012.
YBS revenues from the product during the period totalled £18.6 million. For CSI the figure was £19 million.