The investigation, being led by the German Federal Cartel Office, is looking at Amazon's relationship with third-party sellers on its platform. It began around April and comes after the Cartel Office received a number of complaints. A spokesperson for the regulator told CNBC that it is "not up to a private platform to be a price regulator or the price police". Amazon is using "unknown mechanisms" to regulate sellers on its platform, they added.

German newspaper Frankfurter Allgemeine Zeitung reported: "We are currently investigating whether and how Amazon influences retailers' pricing on the marketplace."

Cartel Office president Andreas Mundt told Frankfurter Allgemeine Zeitung that Amazon had provided it with a statement after it asked the e-commerce giant a number of questions. The responses in the statement are now being considered. The Cartel Office, which has the power to fine companies hundreds of millions of euro, declined to comment on how long the probe will take.