Vanguard Group Inc. is expanding free trading to individual stocks and options, making it the latest Wall Street brokerage to cut commission fees.

Effective Jan. 2, the Pennsylvania-based money manager will no longer charge its brokerage clients a commission fee to trade stocks and options online.

An early pioneer in the low-cost investing business, Vanguard already offers its clients commission-free trading when dealing with the company's own mutual funds or nearly any exchange-traded fund in the industry. Vanguard's expansion in free trading comes almost three months after Charles Schwab Corp. kicked off a heated pricing war in the online brokerage business. Other companies that subsequently rolled out commission-free trading on their platforms include TD Ameritrade Holding Corp., E*TRADE Financial Corp. and Fidelity Investments.

The move to zero has been in the works for years now. New competitors like Robinhood Markets Inc. have challenged the legacy brokerages' business models, while investors have clamored for lower fees and better technology.

However, the departure from commissions has raised questions about how brokerages on Wall Street will make up the lost revenue.

Those companies could become more reliant on a source of income known as payment for order flow, through which brokerages like Schwab, TD Ameritrade and E*TRADE sell their clients' orders in aggregate to sophisticated and speedy traders who then execute the orders themselves. Others have raised concerns that brokers are not providing appropriate yields to their customers in their bank sweep accounts.

Vanguard says its default brokerage sweep option has a higher yield than most bank sweep accounts and that it does not accept payment for order flow related to stocks and ETFs.

"The continued reduction and elimination of fees across the investment industry is a positive development for investors and one that Vanguard has long championed," Karin Risi, managing director of Vanguard's retail investor group, said in a statement. "However, as we move to an environment in which 'zeros' dominate the headlines and explicit fees become implicit, we encourage investors to look more deeply at the total cost picture."