The European Commission has fined Google €2.42bn for breaching EU competition rules.
The commission says Google abused its market dominance as a search engine by giving an illegal advantage to another Google product, its comparison shopping service.
The commission has ordered Google to end the alleged practice within 90 days or face penalty payments of up to 5% of the average daily worldwide turnover of Alphabet, Google's parent company.
In a statement, the commission accused Google of denying other companies the chance to compete on the merits and to innovate, and denying European consumers a genuine choice of services and the full benefits of innovation.
The commission notes that Google entered the comparison shopping space in Europe in 2004, with a product initially called Froogle and later renamed Google Product Search in 2008.
Since 2013 it has been called Google Shopping.
It allows consumers to compare products and prices online and find deals from online retailers of all types, including online shops of manufacturers, platforms such as Amazon and eBay, and other re-sellers.
The commission statement holds that from 2008, Google changed its strategy in order to push its comparison shopping service, which, it states, relies to a large extent on traffic to be competitive.
The strategy relied on Google's dominance in general internet search instead of competition on the merits in comparison shopping markets.
"Google has systematically given prominent placement to its own comparison shopping service: when a consumer enters a query into the Google search engine in relation to which Google's comparison shopping service wants to show results, these are displayed at or near the top of the search results.
"Google has demoted rival comparison shopping services in its search results: rival comparison shopping services appear in Google's search results on the basis of Google's generic search algorithms.
"Google has included a number of criteria in these algorithms, as a result of which rival comparison shopping services are demoted," the statement says.
The statement said that in some cases rival shopping comparison services only appear on page four of the search results.
The commission argues that more traffic leads to more clicks and generates revenue.
More traffic also attracts more retailers that want to list their products with a comparison shopping service.
"Given Google's dominance in general internet search, its search engine is an important source of traffic for comparison shopping services," the statement says.
In a statement, Commissioner Margrethe Vestager said: "Google has come up with many innovative products and services that have made a difference to our lives.
"That's a good thing. But Google's strategy for its comparison shopping service wasn't just about attracting customers by making its product better than those of its rivals.
"Instead, Google abused its market dominance as a search engine by promoting its own comparison shopping service in its search results, and demoting those of competitors."
Ms Vestager added: "What Google has done is illegal under EU antitrust rules. It denied other companies the chance to compete on the merits and to innovate.
"And most importantly, it denied European consumers a genuine choice of services and the full benefits of innovation."
Google has said it "respectfully disagrees" with the fine, and is considering an appeal.
A Google spokesperson said: "When you shop online, you want to find the products you're looking for quickly and easily. And advertisers want to promote those same products.
"That's why Google shows shopping ads, connecting our users with thousands of advertisers, large and small, in ways that are useful for both.
"We respectfully disagree with the conclusions announced today."