Regulators say the securities industry has inadvertently withheld millions of dollars in volume discounts from mutual fund investors, and ordered brokerages to return the money immediately, with interest.

The NASD said most of its members made the mistake, which amounted to failing to notice when clients bought enough of a fund to qualify for a discounted commission. The industry refers to the threshold as a "breakpoint."

The ruling followed an NASD order that member firms examine their accounts to see if they'd been overcharging clients.

"A preliminary analysis of the data obtained as a result of the self-assessment indicates that most members did not uniformly deliver appropriate breakpoint discounts," the NASD said in a release. "The degree to which firms applied the appropriate discount varied.

"It is imperative that member firms make appropriate refunds to customers who did not receive discounts for which they were eligible and that members properly account for overcharge liabilities," the NASD said.

While not a willful deception, the order is the latest in a series of black eyes for mutual funds and the brokerages that sell them. Earlier this month, Massachusetts sued

Morgan Stanley

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(MWD)

for allegedly using improper incentives to get brokers to push certain funds, and the

Securities and Exchange Commission

is in the process of writing new rules to expand the fund industry's disclosure obligation.

In its ruling, the NASD said the process of reimbursing affected clients should be undertaken at once.

"NASD expects that members will make refunds to customers expeditiously where they are aware that customers did not receive the sales load discount to which they were entitled," it said. "Additionally, NASD expects members to make refunds to any customers who come forward and assert that they did not receive all applicable breakpoint discounts."

A firm that fails to return the money quickly faces disciplinary action "separate and apart from any disciplinary actions that may result from the initial failure to deliver breakpoint discounts," the NASD said.