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BRUSSELS (Reuters) - European Union regulators are checking whether U.S. online retailer Amazon AMZN.O is using merchants' data in a way that stifles competition, Europe's antitrust chief said on Wednesday.

The comments by European Competition Commissioner Margrethe Vestager come as the world’s largest online retailer faces calls for more regulatory intervention and even its potential break-up because of its sheer size.

Vestager said the issue was about a company hosting merchants on its site and at the same time competing with those same retailers by using their data for its own sales.

“We are gathering information on the issue and we have sent quite a number of questionnaires to market participants in order to understand this issue in full,” Vestager told a news conference.

“These are very early days and we haven’t formally opened a case. We are trying to make sure that we get the full picture.”

Seattle-based Amazon declined to comment. Half of all items sold on its site globally come from small- and medium-sized businesses. Last year, third party sellers in Britain sold more than 2.3 billion pounds ($3.03 billion) worth of products outside the country while in Germany, they sold 2.1 billion euros ($2.45 billion) outside Germany.

Vestager has the power to fine companies up to 10 percent of their global turnover for breaching EU antitrust rules.