Brussels could escalate a legal challenge to Berlin's planned road toll in April, an EU source said today, as the two sides remain at loggerheads over the German transport minister's pet project.

The executive European Commission launched its challenge, known as an infringement proceeding, against Germany last June.

It argues that Germany's plans to charge foreign drivers up to 130 euros ($145) a year to use its Autobahn motorways but give German drivers a corresponding reduction in automobile taxes contravene EU rules for equal treatment.

The Commission may issue an opinion in April in which it would lay out reasons it believes the toll breaches European Union law, an EU source said.

The step after that would be a referral to the European Court of Justice though the person said there has been no final decision and things could still change.

German Transport Minister Alexander Dobrindt has repeatedly rejected assertions that the toll is discriminatory and said it would generate some 500 million euros for the state each year, to be invested in transport infrastructure.

Initially meant to be introduced in 2016, the toll is on hold until Germany and the EU find an agreement.

European Commission spokesman Jakub Adamowicz said it had requested further information from Germany in December and was assessing what Berlin had provided before deciding on the next steps, which might be a reasoned opinion.

Dobrindt and his Bavarian Christian Social Union (CSU) party have long wanted foreign motorists to pay tolls on motorways because they say it is unfair that foreigners travel for free in Germany while Germans have to pay tolls in neighbouring countries such as Austria, Switzerland and France.

While the toll is a pet project of the CSU, Merkel's bigger Christian Democrat (CDU) sister party and the co-governing centre-left Social Democrats (SPD) were long sceptical.

They finally agreed to the measure provided it conformed with EU rules.