The latest FCA figures revealed the extent to which people were missing out on guaranteed annuity rates, says the firm.
An estimated £3bn of pensions with guaranteed annuity rates were not taken up since the Pension Freedoms in 2015, with nearly six in 10 people who had valuable guarantees giving them up when it came to retirement.
Nine in 10 of those simply cashed their pension in.
A total of 66,546 of the pensions accessed last year had a guaranteed annuity rate.
Typical guaranteed annuity rates are 8-10%, compared to current rates of around 5.5% at age 65.
Nathan Long, senior analyst at Hargreaves Lansdown, said: “Older style pensions often have valuable guarantees built in, in fact more than one in 10 pensions being accessed have them.
“It’s easy to miss out as you try and wade through the pension paperwork when it comes to retirement.
“Today’s annuity rates of around 5.5% at age 65 are dwarfed by guaranteed rates which often are in the region of 8 or 10%.
“To get that kind of rate you need to be in your 80s or in very poor health.