Amazon denied any wrongdoing and said it supports thousands of businesses in Europe.
In Brussels, Amazon has been gearing up for a legal fight. It has hired a team of lawyers and economists, including several who in the past were encouraging tougher enforcement against Google and Microsoft.
“We disagree with the preliminary assertions of the European Commission and will continue to make every effort to ensure it has an accurate understanding of the facts,” the company said in a statement. “No company cares more about small businesses or has done more to support them over the past two decades than Amazon.”
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, Amazon has information on about 800,000 active sellers using its platform, covering more than 1 billion products, 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.