Brussels regulators were trying to find out whether the firms were guilty of "restrictive business practices and/or abuse of a dominant market position," particularly concerning access to pipelines and storage facilities, commission spokesman Jonathan Todd informed.

He added that the raid as such did not suggest the companies were "guilty of any anti-competitive behaviour."

The probe comes as part of the EU executive's attempt to crackdown on monopoly practices in the bloc's energy sector, announced by competition commissioner Neelie Kroes in February.

Jean-Francois Bellis, professor of competition law at Brussels' Free University commented "The commission has made greater competition in the energy sector a priority but it is finding that prices aren't moving. So it's normal that it takes action," according to AFP.

Meanwhile, the British energy market regulator, Ofgem, hailed Wednesday's move.

"The commission is rightly baring its teeth," said Ofgem chief Sir John Mogg, adding "It is now demonstrably serious about tackling anti-competitive practices in the European energy markets. And it needs to be," UK daily Guardian reported.

Under EU rules, the commission could fine companies it finds guilty of abusing their market power up to 10 percent of their total annual turnover.

But it will probably take several months before Brussels regulators decide to file any charges against the firms targeted by the raids.