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.

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

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.