The European Union's ombudsman has opened an investigation into claims by O2 that the European Commission denied it its rights of defence in an ongoing investigation into roaming charges.

"The aim of the inquiry is to determine if there has been maladministration by the Commission," said Nikiforos Diamandouros, the ombudsman for the EU's institutions. He has asked the Commission to respond to the company's allegations by December 31.

The Commission has been investigating three of Europe's biggest mobile phone companies, including O2, for more than seven years. It issued formal antitrust charges against Vodafone and O2 in July 2004, and against Deutsche Telekom AG and Vodafone in February 2005.

The groups were accused of abusing their dominant market position by demanding "unfair and excessive" roaming charges, the fees for making and receiving mobile phone calls abroad.

In its complaint to the ombudsman, O2 accused the Commission of failing to provide proper access to files and information, as well as failing to allow a reasonable time for responding to allegations. Further complaints include the infringement of O2's right to a supplementary statement of objections and of the right to be heard properly, Diamandouros said.

The Commission believes it has conducted the roaming investigation correctly. "We don't think O2's complaint is well founded," said Jonathan Todd, the Commission's spokesman on competition matters. "We scrupulously respected the company's right of defence," he added.

The ombudsman cannot overturn a Commission decision, but his rulings carry political weight. A ruling against the Commission while its probe of roaming charges continues would be embarrassing for Europe's top competition watchdog. It could also allow O2 to lodge a formal appeal against the Commission at the European Court of First Instance in Luxembourg.

O2 representatives weren't immediately available to comment, but last month when the company submitted its complaint to the ombudsman it said it had become "increasingly exasperated with the Commission's conduct of the case and its disregard for due process and O2's procedural rights."