Google has illegally built “monopoly power” with its web advertising business, a federal judge in Virginia ruled (PDF) on Thursday, siding with the Department of Justice in a landmark case against the tech giant that could reshape the basic economics of running a modern website.

Google illegally acted to preserve “monopoly power” in the ad-tech business, a federal judge ruled Thursday. It’s the second antitrust case brought by the U.S. government that has gone against the internet giant in the past year.