German authorities had noted price differences that ranged from 25 percent to 45 percent, the office said in a statement on Wednesday, March 5.

The cartel office would therefore check whether recent German gas price hikes were justified, it added.

"Horribly weak" competition

Office head Bernhard Heitzer said preliminary results of the probe suggested that competition was "still horribly weak" in the gas sector.

"From what we know now, an entire series of companies have raised gas prices to a level that they could never demand in a functioning, competitive economy," he said.

The price increases concerned four million clients and around 20 percent of the market, the office said.

A spokesman contacted by the AFP news service declined to provide the names of companies under investigation, saying only that national and regional firms were concerned.

In all, 770 companies supply gas to households and companies in Germany, of which around 30 are active throughout the country.

Some don't pass on savings

Regional anti-cartel offices had also launched their own investigations into gas price increases.

Heitzer said a number of companies were raising gas prices in a way that would be impossible to do in a normal competitive situation.

"Even in places where the network fees are low, that price advantage isn't necessarily passed on to clients; the savings appear to be used to boost the company's value in another area," Heitzer added.