The European Commission began a formal antitrust investigation into Amazon's e-book business on Thursday, opening yet another front in mounting European Union scrutiny of America's global tech giants. The investigation adds to the pressure on the online retailer in Europe, where it is already being investigated for the low tax rates it pays in Luxembourg. The commission said it would look in particular into certain clauses included in Amazon's contracts with publishers. These clauses, it said, required publishers to inform Amazon about more favourable or alternative terms offered to Amazon's competitors, a means to ensure Amazon was offered terms at least as good as those of its competitors. The commission said this could violate EU antitrust rules that prohibit abuses of a dominant market position and restrictive business practices. However, Amazon said it was confident its agreements with publishers were legal and in the best interests of readers. "We look forward to demonstrating this to the commission as we cooperate fully during this process," it said. In April, the EU executive opened a formal investigation into web giant Google's business practices regarding web search results, and later this year will conduct a sweeping review of the behaviour of online platforms. Antitrust officials are also probing the European tax affairs of Apple, as well as the Starbucks chain, putting US multinationals squarely at the centre of concerns over low corporate tax deals arranged with small EU states. US President Barack Obama warned earlier this year against Europe turning to protectionism to the detriment of the US tech sector.