Shares of Google (GOOG.US,GOOGL.US) fell more than 4.4% after the company lost an antitrust case related to its search business.

On Monday, a federal judge ruled that Google (GOOG.US, GOOGL.US) had maintained monopoly power in online search, a decision that could fundamentally alter the way the tech giants do business and may have implications for Apple (AAPL.US), which makes money from Google being the default search engine on iPhones.

Amit P. Mehta, a judge in the U.S. District Court for the District of Columbia, wrote in his nearly 300-page decision that “Google is a monopolist and acts to maintain its monopoly.” He said that Google maintained its monopoly power in two markets (general search services and general display advertising) by signing exclusive distribution agreements with browser manufacturers, smartphone makers and wireless carriers.

According to court documents, Google’s business partners typically received a portion of the advertising revenue generated by searches from the default search points on browsers or devices. “Google’s distribution agreements are exclusive and have anticompetitive effects, and Google exercises its monopoly power by charging supracompetitive prices for general search text ads,” Mr. Mehta wrote. “This behavior enables Google to obtain monopoly rents.”

The ruling could also affect Apple, which makes money from Google being the default search engine on iPhones.

Shares of Alphabet and Apple fell on Monday, and both fell further after the ruling was announced, with Alphabet closing down more than 4.4 percent and Apple down 4.8 percent. Google said it planned to appeal the ruling, and Apple did not immediately comment on the ruling.

Merrick Garland, the United States attorney general, said in a statement after the ruling, “This victory for the Department against Google is a historic victory for the American people. No company — no matter how large or influential — can stand above the law. The Department will continue to vigorously enforce our antitrust laws.”