The Financial Conduct Authority (FCA) has been urged to act as platforms have revealed they receive millions in interest earned on client cash accounts.
Several experts have called on the regulator to impose and enforce stricter disclosure requirements on platforms, saying they were concerned clients did not know how much of their accrued interest was being kept from them.
The issue concerns interest accrued on money held in bank accounts from dividend proceeds or savings used to make regular investments.
It emerged in January that Hargreaves Lansdown generated £33m of revenue in the second half of last year from interest accrued on clients' accounts.
This was almost as much as the £34m income the Bristol-based business received from annual charges on its own funds.
But Hargreaves Lansdown is not alone. Research carried out by Financial Adviser showed the practice was widespread among platforms.
Financial Adviser found almost all platforms retain some of the interest earned, and some keep all of it.
Abraham Okusanya, founder of consultancy firm Finalytiq, said the regulator should approach the issue in the same way it clamped down on interest skimming by Sipp providers, where rules were introduced to require firms to disclose that they keep some of the interest.
Mr Okusanya said platform's keeping some of the cash is "poor practice" but within the current rules, and said the regulator's policy of requiring disclosure is "ineffective" and said the regulator "should get involved."
Mr Brodie said the FCA has "done nothing" on the issue despite being aware of it. He said "without a doubt" the FCA should be doing more.
There are already safeguards in place under the Financial Conduct Authority's Cass rules, which require firms to pay a retail client interest earned on client money unless it has otherwise notified the client in writing.
Firms are also required to disclose the costs and charges information associated with the platform.
But these rules could be strengthened. As part of its platform market study the regulator asked how disclosure requirements could be made more effective at warning consumers of the costs and charges associated with holding cash balances and whether there were more effective alternatives.
A spokesman for Hargreaves Lansdown said the firm was abiding by "very strict regulatory rules, account reconciliations, statements and reporting".
He said: "We place this [money] with various banks and the interest earned is Hargreaves Lansdown's. We share some of the interest with the client.
"The interest rate paid to clients is competitive and reflective of the nature of the account. Not all platforms pay interest, we do."
In client documentation Hargreaves Lansdown states that it "does not charge" for holding cash, but that it "receives all the cash" paid in interest, and then "separately" pays interest to clients "at rates determined by us."
Mike Barrett, consultant at the Lang Cat, said the wording certainly complied with the regulation, but he added: "The FCA have posed a number of questions to platforms regarding how they handle cash as part of the platform market study.