The European Commission has opened a formal investigation into possible anti-competitive behaviour of online retailer, Amazon.
It relates to whether Amazon's use of sensitive data, from independent retailers who sell on its marketplace, is in breach of EU competition rules.
The retailer has a dual role: it sells products on its website as a retailer, and it provides a marketplace where independent sellers can sell products directly to customers.
The European Commission says: "When providing a marketplace for independent sellers, Amazon continuously collects data about the activity on its platform.
"Based on the commission's preliminary fact-finding, Amazon appears to use competitively sensitive information about marketplace sellers, their products and transactions on the marketplace."
The commission says it will now launch an in-depth investigation.
It will look into standard agreements between Amazon and marketplace sellers - in particular, focusing on whether and how the use of accumulated marketplace seller data by Amazon as a retailer affects competition.
It will also examine the role of data in the selection of the winners of the 'Buy Box', which is displayed prominently on Amazon and allows customers to add items from a specific retailer directly into their shopping carts.
The commission adds: "Winning the 'Buy Box' seems key for marketplace sellers as a vast majority of transactions are done through it."
Competition Commissioner Margrethe Vestager says: "European consumers are increasingly shopping online.
"E-commerce has boosted retail competition and brought more choice and better prices.
"We need to ensure that large online platforms don't eliminate these benefits through anti-competitive behaviour.
"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."
There is no legal deadline for bringing an antitrust investigation to an end.
Earlier this year, search giant Google was fined €1.49bn for breaching EU antitrust rules.