There are good reasons the Justice Department and the Federal Communications Commission should block Comcast’s $45 billion acquisition of Time Warner Cable. The merger will concentrate too much market power in the hands of one company, creating a telecommunications colossus the likes of which the country has not seen since 1984 when the government forced the breakup of the original AT&T telephone monopoly.

The combined company would provide cable-TV service to nearly 30 percent of American homes and high-speed Internet service to nearly 40 percent. Even without this merger and the proposed AT&T-DirecTV deal, the telecommunications industry has limited competition, especially in the critical market for high-speed Internet service, or broadband, where consumer choice usually means picking between the local cable or phone company.

By buying Time Warner Cable, Comcast would become a gatekeeper over what consumers watch, read and listen to. The company would have more power to compel Internet content companies like Netflix and Google, which owns YouTube, to pay Comcast for better access to its broadband network. Netflix, a dominant player in video streaming, has already signed such an agreement with the company. This could put start-ups and smaller companies without deep pockets at a competitive disadvantage.

There are also worries that a bigger Comcast would have more power to refuse to carry channels that compete with programming owned by NBC Universal, which it owns. Comcast executives say that they would not favor content the company controls at the expense of other media businesses.