A judge on Monday ruled that Google’s ubiquitous search engine has been illegally exploiting its dominance to squash competition and stifle innovation, a seismic decision that could shake up the internet and hobble one of the world’s best-known companies.

The highly anticipated decision issued by U.S. District Judge Amit Mehta comes nearly a year after the start of a trial pitting the U.S. Justice Department against Google in the country’s biggest antitrust showdown in a quarter century.

After reviewing reams of evidence that included testimony from top executives at Google, Microsoft and Apple during last year’s 10-week trial, Mehta issued his potentially market-shifting decision three months after the two sides presented their closing arguments in early May.

“After having carefully considered and weighed the witness testimony and evidence, the court reaches the following conclusion: Google is a monopolist, and it has acted as one to maintain its monopoly,” Mehta wrote in his 277-page ruling. He said Google’s dominance in the search market is evidence of its monopoly.  Google “enjoys an 89.2% share of the market for general search services, which increases to 94.9% on mobile devices,” the ruling said.

It represents a major setback for Google and its parent, Alphabet Inc., which had steadfastly argued that its popularity stemmed from consumers’ overwhelming desire to use a search engine so good at what it does that it has become synonymous with looking things up online. Google’s search engine processes an estimated 8.5 billion queries per day worldwide, nearly doubling its daily volume from 12 years ago, according to a recent study released by the investment firm BOND.

Kent Walker, Google’s president of global affairs, said the company intends to appeal Mehta’s findings.

“This decision recognizes that Google offers the best search engine, but concludes that we shouldn’t be allowed to make it easily available,” Walker said. For now, the decision vindicates antitrust regulators at the Justice Department, which filed its lawsuit nearly four years ago while Donald Trump was still president, and has been escalating it efforts to rein in Big Tech’s power during President Joe Biden’s administration.

“This victory against Google is an historic win for the American people,” said Attorney General Merrick Garland. “No company — no matter how large or influential — is above the law. The Justice Department will continue to vigorously enforce our antitrust laws.”

The case depicted Google as a technological bully that methodically has thwarted competition to protect a search engine that has become the centerpiece of a digital advertising machine that generated nearly $240 billion in revenue last year. Justice Department lawyers argued that Google’s monopoly enabled it to charge advertisers artificially high prices while also enjoying the luxury of not having to invest more time and money into improving the quality of its search engine — a lax approach that hurt consumers.

Mehta’s ruling focused on the billions of dollars Google spends every year to install its search engine as the default option on new cellphones and tech gadgets. In 2021 alone, Google spent more than $26 billion to lock in those default agreements, Mehta said in his ruling.

Google ridiculed those allegations, noting that consumers have historically changed search engines when they become disillusioned with the results they were getting. For instance, Yahoo was the most popular search engine during the 1990s before Google came along.

Mehta said the evidence at trial showed the importance of the default settings. He noted that Microsoft’s Bing search engine has 80% share of the search market on the Microsoft Edge browser. The judge said that shows other search engines can be successful if Google is not locked in as the predetermined default option.