The case reinforces the European Union’s role as a leading tech-industry watchdog, even as past investigations of companies like Google have done little to diminish their power. Authorities in Brussels have argued that the biggest tech platforms unfairly use their power to box out competitors, though means like bundling products, charging high fees in app stores and hoarding data.
Ms. Vestager has raised alarms about the “gatekeeper” role of companies like Amazon, Apple, Facebook and Google. The companies have reached such a size, Ms. Vestager has argued, that they are essentially micro-economies, setting rules and policies with little transparency that determine the fate of millions of other businesses that have no choice but to follow along.
About 2.3 million third-party merchants around the world use Amazon to reach customers, including about 37 percent who rely on the company as their sole source of income, according to a United States congressional report published last month. In the European Union, about 800,000 third-party sellers use Amazon, according to the European Commission.
Ms. Vestager has warned the biggest companies will only grow stronger without tougher antitrust enforcement and new regulations, blocking new companies and innovations from emerging.
Next month, the European Commission is expected to unveil a new package of laws that would represent one of the world’s most sweeping set of regulations of the tech industry. It could include rules prohibiting the self-preferencing of products and requiring the biggest companies to share data with smaller rivals.
In the Amazon case, European authorities spent two years investigating the company’s dual role as both a retail store and seller of its own goods. Smaller merchants have long raised concerns that if Amazon sees a particular product selling well on its website, the company would create its own version, sell it at a lower price and then give it better placement on the Amazon website.
The European Commission said those concerns were supported by a review of data on more than 80 million transactions and 100 million products. Ms. Vestager said it showed how Amazon used the data from outside sellers to determine what products to introduce, where to set the price and how to manage of inventory.
Critics of Amazon cheered the European decision.
“Amazon, by using its very powerful position on e-commerce markets and its dual role as both retailer and marketplace, is making it more difficult for independent retailers to compete fairly,” said Agustin Reyna, director of legal and economic affairs at the European Consumer Organization, a group that has been urging regulators to act against Amazon.