The retail brokerage arm of SunTrust Bank has agreed to pay a $1.1 million penalty to settle SEC charges that it recommended more expensive share classes of mutual funds to clients when cheaper shares of the same funds were available, according to an SEC announcement on Thursday.

SunTrust Investment Services collected more than $1.1 million in avoidable fees from more than 4,500 accounts, the SEC said.

The regulator chided the firm for breaching its fiduciary duty to act in clients' best interests by recommending and purchasing costlier mutual fund share classes that charge 12b-1 fees when share class options that did not charge such fees were available.

"SunTrust made self-serving investment recommendations to the detriment of everyday investors who rely on mutual funds to secure their financial futures," Aaron Lipson, associate regional director for Enforcement in the SEC's Atlanta office, said in a statement.

SunTrust began refunding the overcharged fees plus interest to affected clients after the SEC opened an investigation in 2015.

"We addressed the matter on a prospective basis with remedial actions starting in the summer of 2015," Hugh Suhr, a spokesman for SunTrust, said in an email. "Although we believe that our disclosures were in accordance with industry standards, we cooperated fully with the SEC and are pleased to have settled this matter."

The firm agreed to a censure and a penalty totaling $1,148,072 as well as disgorgement plus interest on any leftover amount of the avoidable 12b-1 fees that are being refunded to clients.

It agreed to the sanctions without admitting or denying the SEC's findings.