Google has been fined €2.4bn (£2.1bn) by the European Commission for promoting its own shopping comparison service at the top of search results.
The penalty is the largest the EU regulator has ever handed out to a company that has been accused of distorting the market.
The European Commission’s ruling has also ordered the US online giant to end the practices within 90 days, or face an additional penalty.
If it fails to change the way it operates within that deadline, Google’s parent company Alphabet could have to shell out up to 5% of its average worldwide earnings.
According to its most recent financial report, that amounts to $14m (£10.9m) per day.
Google said it is considering an appeal against the European Commission’s ruling.
Google Shopping displays relevant product images and prices alongside the names of shops they are available from.
The information is labelled as being “sponsored” to show that sellers have paid for those items to appear.
Margrethe Vestager, the European Union’s competition commissioner, said: “What Google has done is illegal under EU antitrust rules.
“It has denied other companies the chance to compete on their merits and to innovate, and most importantly it has denied European consumers the benefits of competition, genuine choice and innovation.”
‘We respectfully disagree’
The decision could set a precedent, following complaints about how Google gives prominence to its own maps and flight price results in its search tools.
A Google spokesman 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.
“We will review the Commission’s decision in detail as we consider an appeal and we look forward to continuing to make our case.”
Any appeal would prolong a probe that has already lasted almost seven years.
The European Commission started its investigation into Google Shopping in late 2010, following complaints from the likes of Microsoft.
Shivaun Raff, chief executive of another of the original complainants, price comparison service Foundem, said: “Although the record-breaking €2.42bn fine is likely to dominate the headlines, the prohibition of Google’s immensely harmful search manipulation practices is far more important.
“For well over a decade, Google’s search engine has played a decisive role in determining what most of us read, use and purchase online.
“Left unchecked, there are few limits to this gatekeeper power.”