Amazon is facing an investigation by the European Commission into its e-book contracts with publishers .

European regulators have opened a formal antitrust investigation into clauses included in Amazon’s contracts with publishers that require them to inform Amazon about more favourable terms offered to the retailer’s competitors. The contracts also require publishers to ensure Amazon is offered terms that are at least as good as those for its competitors.

The commission said it was concerned these clauses may make it more difficult for other e-book distributors to compete with Amazon, reducing choice for consumers.

“Amazon has developed a successful business that offers consumers a comprehensive service, including for e-books. Our investigation does not call that into question,” EU Commissioner in charge of competition policy Margrethe Vestager said. “However, it is my duty to make sure that Amazon’s arrangements with publishers are not harmful to consumers, by preventing other e-book distributors from innovating and competing effectively with Amazon. Our investigation will show if such concerns are justified.”

The e-books probe is Amazon's latest clash with the EU after it was embroiled in an investigation into tax loopholes for multinationals including Apple. Since taking office in November, Vestager has also sent Google a formal antitrust complaint for shutting out rival search engines and signalled a clampdown on possible barriers to e-commerce and digital content including Hollywood studios' pay-TV deals.

Amazon, now the largest distributor of e-books in Europe, helped pioneer the market with the introduction of the Kindle device in 2007.

“Amazon is confident that our agreements with publishers are legal and in the best interests of readers,” the company said in an e-mailed statement. “We look forward to demonstrating this to the commission as we cooperate fully during this process.”

Additional reporting: Bloomberg