O2, the mobile phone operator, has issued a formal complaint against the European Union’s top antitrust regulator, saying Brussels ignored its rights of defence in a long-running probe into allegedly excessive roaming fees.

The unusual move could prove embarrassing for the European Commission, which has been investigating three of the region’s biggest mobile phone companies for more than seven years. It could also further sour relations between the industry and the Commission, which recently proposed legislation forcing mobile operators to slash roaming fees.