European Union regulators took the German government to court over its controversial plan to impose highway tolls on cars, saying the rules discriminate against foreign drivers, Bloomberg reports.

The lawsuit by the European Commission, the EU’s regulatory arm, signals exasperation with German Chancellor Angela Merkel’s administration two years after it announced the move to end free driving on Europe’s biggest national highway network. The step was a concession by Merkel to Bavarian allies in her coalition government who complained about road tolls collected by Austria and Switzerland.

The German road-charging initiative, on hold amid the legal standoff, entails a “de facto exemption” for cars registered in Germany through a reduction in annual vehicle tax, said the commission. The plan also includes “disproportionately high” prices for vignettes of less than a year intended only for cars registered abroad, according to the commission, which sent a final warning to Berlin over the whole matter in April.

The legal case highlights the commission’s commitment to upholding the EU principle that member countries cannot favor their own citizens over people from elsewhere in the European single market.

“When the single-market rights of citizens are being threatened, this is of course something where the commission will act,” Itkonen said.

German Transport Minister Alexander Dobrindt said the commission’s decision to take the matter to the EU Court of Justice in Luxembourg was “good news” and predicted a legal victory for Berlin, which has dubbed the planned road tax an “infrastructure surcharge.”