The Justice Department plans to accuse Google of maintaining an illegal monopoly over search and search advertising in a lawsuit to be filed on Tuesday, the government’s most significant legal challenge to a tech company’s market power in a generation, according to officials at the agency.

In its suit, to be filed in a federal court in Washington, D.C., the agency will accuse Google, a unit of Alphabet, of illegally maintaining its monopoly over search through several exclusive business contracts and agreements that lock out competition, said the officials, who were not authorized to speak on the record. Such contracts include Google’s payment of billions of dollars to Apple to place the Google search engine as the default for iPhones.

The agency will argue that Google, which controls about 80 percent of search queries in the United States, struck agreements with phone makers using Alphabet’s Android operating system to pre-load the search engine on their phones and make it hard for rival search engines to become a replacement. By using contracts to maintain its monopoly, competition and innovation has suffered, the suit with argue.