SEATTLE, United States — Amazon, the world's largest online retailer, could face an EU antitrust investigation within days over its use of merchants' data, a person familiar with the matter said on Wednesday.

The European Commission has been seeking feedback from retailers and manufacturers since September last year, one of several competition enforcers taking a deeper look into Amazon's business practices amidst calls by some for its break-up.

European Competition Commissioner Margrethe Vestager has said the issue is 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.

Merchants have complained about harm caused by Amazon copies of their products.

Politico first reported the investigation last week.

The Commission had been struggling to define the market in which Amazon operates in order to identify where the competitive harm could have been, other sources said. They said the issue was whether to look at Amazon in the overall retail market or in its own niche.

The EU competition enforcer, which can fine companies up to 10 percent of their global turnover, did not immediately respond to a request for comment.

This would not be Amazon's first run-in with the Commission. Two years ago, it was told to pay back taxes of about 250 million euros to Luxembourg because of illegal tax benefits. That same year it settled with the regulator over its distribution deals with e-book publishers in Europe.

Separately, Amazon reached a deal with Germany's antitrust authority on Wednesday to overhaul its terms of service for third-party merchants, who had complained of unfair treatment when selling through the world's biggest online retailer.

By Foo Yun Chee