Germany's competition watchdog said on Wednesday it had launched a probe against five major oil companies into allegations they sought to drive independent filling stations out of business.

"The Federal Cartel Office has launched a procedure against five petrol companies -- Deutsche BP/Aral, ExxonMobil Europe/Esso, ConocoPhilips Germany/Jet, Shell Deutschland, Total Deutschland -- on suspicion they tried to obstruct independent filling stations," the watchdog said in a statement.

"We always pursue such allegations," said watchdog chief Andreas Mundt.

"Deliveries to independent petrol stations must priced fairly so that they can compete against the oligopoly of the five majors," Mundt said.

The cartel office had received a string of complaints from independent petrol stations about the pricing practices of the oil majors, he explained.

In several cases, petrol was sold to the independent operators at higher prices than the five companies demanded from motorists at their own service stations.

In addition, at a number of filling stations, petrol was sold at prices below the cost price.

The cartel office saw the German petrol station market -- where independently operated petrol stations account for around one third -- as being in the hands of an oligopoly. The independents purchase most of their petrol from the five main groups.

It is not the first time that the German anti-trust authorities have sought to break the dominance of the oil majors and has repeatedly said it was monitoring their pricing practices.

"It's a continuous process, but it's not easy because prices fluctuate constantly," a spokesman said.

"We've done a lot of preparatory work before taking this move and the five companies now have to respond and submit documentation by a certain deadline," the spokesman said.

A spokesman for Total's German operations said the group had received a letter from the cartel office and been given until the beginning of May to respond.

"Of course we will cooperate," he said, but added: "I wonder how the cartel office will explain to motorists -- who are constantly complaining of rising prices -- that we're being accused to offering prices that are too low."