Alphabet's Google illegally dominates two markets for online advertising technology, a judge ruled on Thursday, dealing another blow to the tech giant and paving the way for U.S. antitrust prosecutors to seek a breakup of its ad products.

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.

The tech giant may be forced to sell parts of its highly profitable advertising technology unit, disrupting a business with huge power over what consumers see online.