A total of four platforms in the UK profit twice from clients' cash holdings.
Data compiled by consultancy firm the Lang Cat showed Parmenion, James Hay, Novia and AJ Bell all have a policy of not passing on any interest received on client cash to the end client, as well as of charging clients to hold cash.
Novia's results out this week showed retaining interest contributed as much as a third to the profits the platform made in 2018.
The accounts showed the platform made a profit of £5.6m in the year, with about £2m of this being attributed to 'other' income such as interest on client cash.
The issue concerns interest accrued on money held in bank accounts from dividend proceeds or savings used to make regular investments.
Earlier this year rival platform Hargreaves Lansdown, which passes on some of the interest earned to clients, reported a profit of £153m, of which £33m was attributable to client interest, representing a ratio of 21 per cent.
Of the platforms the Lang Cat looked at only six do not charge to hold cash, but the bulk of platforms pass on at least some of the interest they receive on the cash to the client.
The six platforms that do not levy a charge to manage cash are Hargreaves Lansdown, Zurich, Seven IM, Old Mutual Wealth, Funds Network, and Alliance Trust Savings.
Platforms' interest rate retention was first flagged by FTAdviser in 2014, when ten out of 12 platforms stated they retained interest from cash accounts.
Novia’s policy is to pay clients an interest rate of the Bank of England base rate minus 0.75 per cent. As the base rate is presently 0.75 per cent, this means the net amount paid to clients is presently zero.
A representative of Novia said while other platforms do not levy a charge directly for holding cash they either have a higher platform charge or other charges, such as exit fees, it does not have.
They added that as UK interest rates rise, clients will get a better return from their cash holdings.
Representatives of Parmenion, which is owned by Aberdeen Standard Investments, said the company has recently decided to change its policy.
The representative said: "Parmenion reviews the cash interest we pay customers on a regular basis.
"We currently receive different rates from our different banking partners and currently receive c.0.2 per cent. We have recently reviewed our policy and are planning to start paying interest on cash balances from July 1.
"We have always been transparent about our fees and charges, including the interest we pay.
"We fundamentally believe that there are better low risk options for customers than holding large amounts of cash on platform and as such, we proactively review individual cash balances and will take action if the amount held in cash looks too high."