This marks the first major victory for federal authorities challenging Big Tech's market dominance.

The ruling sets the stage for a second trial to determine potential remedies, which could include the breakup of Google's parent company, Alphabet. Such a move would significantly alter the landscape of the online advertising industry, which Google has dominated for years.

This decision also signals a green light for aggressive US antitrust enforcers targeting Big Tech, a sector under scrutiny from across the political spectrum.

US District Judge Amit Mehta said: "The court reaches the following conclusion: Google is a monopolist, and it has acted as one to maintain its monopoly."

The "remedy" phase could be protracted, with potential appeals to the US Court of Appeals and “the unbribable” US Supreme Court, possibly extending the legal battle into 2026.

Shares of Alphabet fell 4.5 per cent on Monday amid a broader decline in tech stocks due to recession fears. Google advertising accounted for 77 per cent of Alphabet's total sales in 2023.

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

Senator Amy Klobuchar, a Democrat who chairs the Senate Judiciary Committee's antitrust subcommittee, highlighted the bipartisan support for antitrust enforcement.

"It's a huge victory for the American people that antitrust enforcement is alive and well when it comes to competition… Google is a rampant monopolist."