The EU has announced legal action against seven nations over the way they handled the policing of car emissions, amid a backlash over the VW diesel scandal.
Officials in Brussels said Germany, Britain, Spain and Luxembourg were accused of not imposing the same kind of penalties VW faced in the United States over its use of illegal software to mask emissions of nitrogen oxide (NOx) on tests.
The European Commission has also taken further issue with Berlin and London, claiming both refused to share details on breaches of EU emissions laws uncovered in national investigations.
The other nations' actions under the spotlight are the Czech Republic, Lithuania and Greece.
Commissioner for Industry, Elzbieta Bienkowska, said that while primary responsibility for emission compliance lay with vehicle producers, "national authorities across the EU must ensure that car manufacturers actually comply with the law."
Germany argues EU rules in this area are poorly framed but Brussels insists it is national watchdogs with the power to approve new cars and police them across the 28-nation bloc.
Britain is bringing in 'real world tests' for emissions next year because laboratory procedures fail to give an on-the-road measure of fuel economy and exhaust levels.
The UK's Department for Transport said in April that none of the 37 top-selling diesel vehicles met legal limits when tested on the road.
While tougher laws in the US have so far secured financial commitments from VW topping $15bn, the company has consistently refused to pay compensation to owners in the EU - 1.2 million of them in the UK alone. It has pledged fixes for all vehicles by late next year.
VW remains the subject of several criminal and regulatory inquiries.