The offices of Eon, RWE, Gaz de France and several other dominant European energy groups have been raided as part of a sweeping European Commission antitrust inquiry into alleged competition abuses in the electricity and gas sector.

The raids on more than 20 sites in Germany, France, Italy, Austria, Belgium and Hungary were the most dramatic move so far in the commission's campaign to prise open the European Union's energy markets.

Some of the companies are involved in the energy industry in Ireland.

RWE, for example, is the operator of the Tynagh energy plant in Co Galway.

They reflect the regulator's belief that former state monopolies and other dominant groups bear much of the blame for the poor state of competition in the European energy market.

The commission's high-profile attack on some of the most powerful companies in the EU illustrates the deep concern over the functioning of the pan-European energy market in Brussels and national capitals.

The continent's growing dependency on gas supplies from an increasingly assertive Russia, consumer discontent over spiralling fuel costs, and the current round of consolidation among European energy companies mean the sector has been catapulted to the top of the EU agenda.

The raids, on Tuesday, followed suspicions that the targeted groups had restricted "access to pipeline and storage facilities" and of "concerted practices between incumbents that can be described as market-sharing", a commission spokesman said.

Groups that admitted being raided were Eon and RWE of Germany, Gaz de France, Distrigas and Fluxys of Belgium and OMV of Austria.

All the companies said they were co-operating fully with the investigation.

The commission also announced yesterday it had raided several electricity companies in Hungary.

The companies are suspected of "excluding competitors from the wholesale electricity market by entering into long-term power purchase agreements and import contracts underpinned by long-term reservation of capacity on interconnectors", a Brussels spokesman said.

If they are found to have broken competition rules, the groups targeted by Brussels face fines of up to 10 per cent of their worldwide annual turnover. Though the commission often settles for lower fines, some repeat offenders have been hit recently with the maximum penalty.

The raids are closely linked to a much broader commission inquiry into the energy sector, the first results of which were released in February.