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

It was the second time in a year that a U.S. court found that the company had acted illegally to remain dominant.

Google illegally monopolized some online advertising technology markets, according to a federal judge, whose ruling marked the latest antitrust setback for the company and a challenge to its main source of revenue.