Google has violated US antitrust law with its search business, a federal judge ruled Monday, handing the tech giant a staggering court defeat with the potential to reshape how millions of Americans get information online and to upend decades of dominance.

“After having carefully considered and weighed the witness testimony and evidence, the court reaches the following conclusion: Google is a monopolist, and it has acted as one to maintain its monopoly,” US District Judge Amit Mehta wrote in Monday’s opinion. “It has violated Section 2 of the Sherman Act.”

The decision by the US District Court for the District of Columbia is a stunning rebuke of Google’s oldest and most important business. The company has spent tens of billions of dollars on exclusive contracts to secure a dominant position as the world’s default search provider on smartphones and web browsers.

Those contracts have given it the scale to block out would-be rivals such as Microsoft’s Bing and DuckDuckGo, the US government alleged in a historic antitrust lawsuit filed during the Trump administration.