EU Competition Commissioner Margrethe Vestager said in a statement on Wednesday: "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.”

Vestager gave the first indications of such a probe in September last year when she announced that the EU had started taking a close look at Amazon's business practices.

At the time, she was at pains to emphasise that the investigation was only at the stage of collecting data. "The question that people have been asking increasingly and we've also seen it ourselves in the market is you have these platforms that have sort of dual purpose," she said.

The EC's Wednesday statement pointed out that Amazon's platform both sold products as a retailer and also allowed other retailers to sell products directly to consumers.

"When providing a marketplace for independent sellers, Amazon continuously collects data about the activity on its platform," the EC statement said.

"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."

There is no legal deadline for the investigation to be concluded.