The European Union is launching an antitrust investigation into how Amazon uses data from independent retailers who use its marketplace.
Margrethe Vestager, the European Commission's top antitrust official, has long warned a probe of the giant online retailer was coming.
In a statement on Wednesday, Vestager said e-commerce had boosted choice for consumers and reduced prices.
"We need to ensure that large online platforms don't eliminate these benefits through anti-competitive behavior," she said. "I have therefore decided to take a very close look at Amazon's business practices and its dual role as marketplace and retailer, to assess its compliance with EU competition rules."
The Commission will look into standard agreements between Amazon and the sellers, and whether and how the seller data is used by Amazon unfairly. The investigation will also look into the selection of the "Buy Box" on Amazon's search results.
It's not clear how long the probe will take.
In a statement, Amazon said: "We will cooperate fully with the European Commission and continue working hard to support businesses of all sizes and help them grow."