A US judge ruled on Monday that Google acted illegally to suppress competition and maintain a monopoly on online search and related advertising. This landmark decision deals a significant blow to Alphabet, Google’s parent company, and could reshape the business practices of major technology firms.

Google was sued by the US Department of Justice in 2020 for controlling about 90% of the online search market. This lawsuit is one of several filed against big tech companies as US antitrust authorities seek to enhance competition in the industry. The case has been described as an existential threat to Google and its dominance in the search and online advertising sectors.

The penalties Google and Alphabet will face as a result of the decision are yet to be determined. Fines or other remedies will be decided in a future hearing, and the government has requested “structural relief,” which could theoretically mean the break-up of the company.

In his decision, US District Judge Amit Mehta stated that Google paid billions to ensure it remained the default search engine on smartphones and browsers. “Google is a monopolist, and it has acted as one to maintain its monopoly,” Judge Mehta wrote in his 277-page opinion.

Alphabet announced plans to appeal the ruling. “This decision recognises that Google offers the best search engine but concludes that we shouldn’t be allowed to make it easily available,” the company said in a statement.

US Attorney General Merrick Garland hailed the ruling as a “historic win for the American people.” He emphasized that “no company – no matter how large or influential – is above the law,” and the Justice Department will continue to enforce antitrust laws vigorously.

Federal antitrust regulators have filed other pending lawsuits against Big Tech companies, including Meta Platforms (which owns Facebook), Amazon.com, and Apple Inc., accusing them of operating unlawful monopolies.

Monday’s ruling followed a 10-week trial in Washington DC, where prosecutors accused Google of spending billions annually to ensure its pre-installation as the default search engine on platforms by Apple, Samsung, Mozilla, and others. The US said Google typically pays more than $10bn (£7.8bn) a year for this privilege, securing a steady stream of user data that maintains its market hold. Prosecutors argued that this practice prevented other companies from competing meaningfully.

“The best testimony for the importance of defaults is Google’s cheque book,” argued Department of Justice lawyer Kenneth Dintzer during the trial.

Google’s search engine generates significant revenue for the company, thanks largely to advertising displayed on its results pages. Google’s lawyers defended the company by arguing that users are attracted to their search engine because of its usefulness and that Google invests in making it better for consumers. “Google is winning because it’s better,” said Google’s lawyer John Schmidtlein during closing arguments earlier this year.

Schmidtlein also argued that Google still faces intense competition, not only from general search engines like Microsoft’s Bing but also from specialized sites and apps that people use to find restaurants, airline flights, and more.

In his ruling, Judge Mehta concluded that being the default search engine is “extremely valuable real estate” for Google. “Even if a new entrant were positioned from a quality standpoint to bid for the default when an agreement expires, such a firm could compete only if it were prepared to pay partners upwards of billions of dollars in revenue share,” Judge Mehta wrote.

Another case against Google over its advertising technology is scheduled for trial in September. In Europe, Google has already been fined billions in monopoly cases.