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The European Commission (EC) has opened a formal antitrust investigation into Amazon's e-book business.

The EC will look into clauses in Amazon's contracts that require publishers to inform the US firm about deals they have with its rivals.

Amazon, which owns the Kindle e-reader, is already under pressure over low tax rates it pays in Luxembourg.

New EC Competition Commissioner Margrethe Vestager has begun looking into several US giants.

Her department is probing the tax affairs of Apple and Starbucks, and in April she launched a formal investigation into Google's online business practices.

She said on Thursday that the agreements Amazon strikes with publishers could make it more difficult for other e-book distributors to compete.

Ms Vestager : "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."

Her tougher stance against US multinationals has drawn criticism that she has been targeting American businesses, although she denied this in a ´óÏó´«Ã½ interview earlier this month.

However, President Barack Obama, whose administration is negotiating a new transatlantic free trade deal with the 28-nation European Union, warned earlier this year against Europe turning to protectionism to the detriment of the US technology sector.