EU anti-trust regulators have opened an investigation into Amazon after allegations that it misuses "sensitive data" from independent retailers who sell on the online giant's website.
Anti-trust commissioner Margrethe Vestager said she would take "a very close look" at its business practices.
The European Commission said it would examine Amazon's standard contracts with marketplace sellers.
It will also look at how data is used to allocate prominent slots on pages.
The Commission said Amazon continuously collected data about the activity on its platform.
"Amazon appears to use competitively sensitive information - about marketplace sellers, their products and transactions on the marketplace," the Commission added.
Ms Vestager said she wanted to assess Amazon's compliance with EU competition rules in its dual role as retailer and marketplace.
The in-depth probe has no deadline. Ms Vestager, who has been responsible for a series of EU investigations of US tech giants, is due to complete her mandate on 31 October.
An Amazon spokesperson said: "We will co-operate fully with the European Commission and continue working hard to support businesses of all sizes and help them grow."
Amazon launched Marketplace in the US in 2000 and in the UK in 2002. It allows the online retailer to offer a wider range of products without having to stock them all itself.
For their part, third-party retailers benefit from Amazon's brand, exposure and reach.
According to some estimates, sales by independent retailers on Amazon account for about 30% of Amazon's annual sales.