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.

SAN FRANCISCO (Reuters) - 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 advertising products.

A federal court in Alexandria ruled that Google’s advertising technology unit is an illegal monopoly, in the second of two Justice Department antitrust cases against the tech giant. The decision by U.