The UK Financial Conduct Authority has fined Credit Suisse International and Yorkshire Building Society for promising unrealistic returns to investors who had limited knowledge of markets.
The FCA said it has fined Credit Suisse International £2.4m and Yorkshire Building Society £1.4m.
This was the watchdog's second and third biggest fines for marketing failures which related to investments totalling £797m.
"These promotions were a serious breach of the requirement to be clear, fair and not misleading," the watchdog's director of enforcement, Tracey McDermott, said.
It was also the first time that the watchdog, launched in April 2013 with a specific remit to protect consumers after a string of mis-selling scandals spanning decades, has fined the producer and distributor of a product at the same time.
Credit Suisse was telling customers that its Cliquet deposit product provided capital protection and a guaranteed minimum return, with the apparent potential for significantly more if Britain's FTSE 100 share index performed consistently well.
Almost 83,800 customers invested a total of £797.4m in the product, with YBS thedistributor responsible for approximately 75% of the total amount invested.
The FCA said the probability of achieving only the minimum return was 40-50%, and the probability of achieving the maximum return was close to zero percent.
Both firms have agreed to contact customers who bought the product between November 2009 and June 2012 to offer the chance of exiting it without penalty and, where applicable, receive an interest payment.
After concerns were raised by Britain's consumer group Which? and others in September 2010, Yorkshire Building Society changed its promotional literature but continued to give anunfair impression of the likelihood of achieving maximum returns, the FCA said.
Credit Suisse also reviewed its literature but decided not to change the brochure significantly.
Both firms obtained a 30% discount on their fines after agreeing to settle with the FCA at an early stage