More than a third of investors with an investment manager or IFA did not know how much they were charged last year, according to research by Nutmeg.
Meanwhile more than a quarter - 28 per cent - of those customers who knew what they were paying felt they were paying an unreasonable amount.
On average, customers reported paying 1.69 per cent a year of their overall investments.
According to the research, the mean total value of investments is £160,000 - which means these people are paying £2,704 annually in fees.
Nick Hungerford, chief executive of Nutmeg, said: “For those customers that don’t know how much they are paying or those that are reluctant to check, there is a huge cost to inaction.
“Investment managers and IFAs are failing customers – they have a duty to give customers clarity not only about what they pay, but on how charges are structured.
“Instead charges are complicated and opaque – to the absurd extent that when Nutmeg wants to find out a given wealth manager’s latest fees, we have to commission third-party researchers to do mystery shopping.”
The research was carried out in late February and early March through 2,002 online interviews with UK adults.
Earlier this month Nutmeg announced it had halved its minimum lump sum investment from £1,000 to £500.
It has also been given FCA approval to offer regulated financial advice.