Brokers could be required to tell investors exactly where they sent a stock order to be filled under a measure the U.S. Securities and Exchange Commission (SEC) is weighing to address complaints that the decisions sometimes aren't in clients' best interests.

The proposal could give investors more insight into whether they are getting the best price when they buy and sell large numbers of shares, according to three people familiar with the matter. Brokers entrusted with orders in the U.S. stock market can choose from dozens of exchanges and private venues. Some money managers such as T. Rowe Price Group Inc. have told regulators that incentives offered by exchanges for attracting orders can put a broker's financial interest at odds with the customer's.

The SEC faces pressure to overhaul trading after Michael Lewis's “Flash Boys” book made the claim that high-frequency traders hurt other investors by learning which shares investors plan to buy, purchasing them, and selling them back at a higher price. The SEC has said it's reviewing every aspect of how stocks are traded, and regulators are trying to identify changes that could be implemented quickly, the people said.