A US judge has ruled Google acted illegally to maintain a monopoly on online searches and related advertising.

Google was sued by the Department of Justice in 2020 for its control of about 90% of the online search market.

US District Judge Amit Mehta noted that Google paid billions to ensure it is the default search engine on smartphones and browsers.

The decision on Monday could reshape how technology giants do business.

“Google is a monopolist, and it has acted as one to maintain its monopoly,” Judge Mehta wrote in his 277-page opinion.

The decision is a significant victory for federal antitrust regulators who have filed other pending lawsuits against Big Tech companies, accusing them of operating unlawful monopolies.

Federal regulators have sued Meta Platforms, which operate Facebook and Whatsapp, Amazon.com and Apple Inc.

The ruling comes after a 10-week trial in which prosecutors accused Google of spending billions of dollars annually to be the default search engine for consumers across platforms.

Doing so, prosecutors said, meant other companies did not have the opportunity or resources to meaningfully compete.

Google's lawyers defended the company by saying that users are attracted to their search engine because they find it useful and that Google is investing to make it better for consumers.

“Google is winning because it’s better,” said Google's lawyer John Schmidtlein during closing arguments earlier this year.

Google has not yet commented on Judge Mehta's ruling.

Another case against the technology company over its advertising technology is scheduled to go on trial in September.