European Competition Commissioner Margrethe Vestager has begun questioning merchants on Amazon's use of their data. The issue, she said, is whether Amazon is using data from the merchants it hosts on its site to secure an advantage in selling products against those same retailers.

Amazon allows third-parties to sell goods on Amazon through its Seller program. The questioning by Vestager appears to be designed to decide whether or not Amazon puts these third party sellers at a disadvantage by using their sales data to boost Amazon's sales.

"These are early days, and we haven't formally opened a case. We are trying to make sure that we get the full picture", Vestager said.

Vestager said the EU was looking into the problem because it came up in a broad e-commerce sector investigation concluded last year, but also because “this is also what a lot of people are talking about by now, so we do the follow-up”.

Vestager noted Amazon’s dual role as a platform for retailers to sell their goods as well as a merchant in its own right. That position makes Amazon privy to data about the customers of retailers it competes with. She can fine companies up to 10 percent of their global turnover for breaching EU antitrust rules. Earlier this year, she levied a record $5 billion fine against Google related to its Android business.

The probe comes as the world's largest online retailer faces growing calls for regulation. Investors and insiders have long cited Amazon's size and reach as reasons to break up the company.

US Attorney General Jeff Sessions was set to meet this month with state officials to discuss antitrust concerns in Silicon Valley, though much of the regulation on Big Tech thus far has come out of Brussels.