The EU's anti-trust regulator opened a formal investigation on Thursday into Amazon's e-book distribution, in the latest Brussels inquiry to target the US-based Internet retailer.

"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," Competition Commissioner Margrethe Vestager said in a statement.

"Our investigation will show if such concerns are justified," she added.

The European Commission, the EU executive's arm, is probing clauses Amazon has with its publishers which may shield the company from competitors, including the right to be informed of more favourable terms.

Brussels is already conducting a separate investigation into Amazon's tax arrangements over its tax deals in Luxembourg, one of a series of such probes targeting major global firms.