The future of Google's advertising business is at stake after a federal judge found the company illegally monopolized parts of it.

The judge ruled that Google monopolized the market for two of three sets of products. It was the second time in a year that a U.S. court found it 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.