The US government has filed charges against Google, accusing the company of violating competition law to preserve its monopoly over internet searches and online advertising.
The lawsuit marks the biggest challenge brought by US regulators against a major tech company in years.
It follows more than a year of investigation and comes as the biggest tech firms face intense scrutiny of their practices at home and abroad.
Google called the case "deeply flawed".
The company has maintained that its sector remains intensely competitive and that its practices put customers first.
"People use Google because they choose to - not because they're forced to or because they can't find alternatives," it said.
The charges, filed in federal court, were brought by the US Department of Justice and 11 other states. The lawsuit focuses on the billions of dollars Google pays each year to ensure its search engine is installed as the default option on browsers and devices such as mobile phones.
Officials said those deals have helped secure Google's place as the "gatekeeper" to the internet, allowing it to own or control the distribution channels for about 80% of search queries in the US.
"Google has thus foreclosed competition for internet search," the lawsuit said. "General search engine competitors are denied vital distribution, scale, and product recognition - ensuring they have no real chance to challenge Google."
It added: "Google is so dominant that 'Google' is not only a noun to identify the company and the Google search engine but also a verb that means to search the internet."
The suit said the deals have hurt the public by damaging search quality in terms of privacy and data protection, reducing choice and thwarting innovation.
Sally Hubbard, who works for the Open Markets Institute, a Washington think tank that has long pushed for more aggressive action against big tech firms, said focusing on Google's search distribution deals was one of the easiest legal cases to make against the company.
On Twitter she said the lawsuit had "been so long coming but it's wonderful to see". I have to hop off - more later - but I just gotta say that from what I've read so far this complaint speaks the truth. It's been so long coming but it's wonderful to see.
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