Fidelity — perhaps not surprisingly — called the combination a poor deal for consumers.

“Unfortunately for investors, the combination of Charles Schwab and TD Ameritrade means they will likely be doubling down on revenue practices that directly disadvantage investors,” said Kathleen Murphy, president of Fidelity Investments’ personal investing business, in a statement.

Several analysts said that the industry had expected E-Trade to be scooped up by a larger player first. The last major deal was three years ago, when TD Ameritrade acquired Scottrade in a $4 billion transaction.

“Consolidation has been an uneven trend in this space,” said Stephen Biggar, director of financial institutions research at Argus Research. “When you get down to a few large players, I think it becomes less likely.”

The deal could have consequences for investors even if they don’t have direct accounts with Schwab or Ameritrade, because of the services the firms provide other financial professionals.

The companies are among the largest service providers to independent registered investment advisers: They hold customer assets, execute and clear trades and handle much of the administrative work that goes along with it.

Schwab is already the largest so-called custodian, with $1.8 trillion in assets managed by registered investment advisers; it controls roughly half of the market, according to Keefe, Bruyette & Woods, an investment bank that specializes in financial services. Ameritrade now ranks as the third-largest player, with up to 20 percent of the market. Fidelity is in second place.

“We think this deal may face somewhat significant antitrust hurdles,” Kyle K. Voigt, an analyst at K.B.W., said in a research note, “depending on how the competitive market is viewed by relevant authorities.”