The European Commission on Thursday referred Britain, France, Germany, Hungary, Italy, and Romania to the EU's highest Court of Justice, saying they failed to respect air quality limits and didn't take appropriate action in time.
The Commission also warned Britain, Germany, Italy and Luxembourg for disregarding car approval rules.
Bas Eickhout of the Greens/EFA group in the European parliament said that "it is shameful that some of Europe's wealthiest countries are dragging their heels on protecting their citizens' health."
In Germany, the EU's biggest economy, the decision adds to pressure on the government after a national court ruled in February that cities can ban diesel cars and trucks to combat air pollution.
Chancellor Angela Merkel's government has scrambled to reassure drivers it would seek to prevent such drastic measures by pushing other ways to reduce urban pollution.
Speaking after a meeting of EU leaders in Sofia, Bulgaria, Merkel dismissed the suggestion that the government hasn't done enough to comply with air quality requirements.
She said her government was giving "unprecedented" support to municipalities to deal with the problem, and added that "we are on a very, very good path."
"The European Commission knows this path, and I think we will make progress very quickly in various areas," Merkel added.
Copyright 2018 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.