Volkswagen emissions: UK and six other nations face legal action
The European Union (EU) has started legal action against seven nations.
Four, including the UK and Germany, are under fire for failing to take action against Volkswagen for cheating emission tests. Member states have two months to respond.
The German car giant has had huge fines in the US over its use of "defeat devices" used to hide true levels of emissions.
More than one million cars in the UK are involved.
Spain and Luxembourg are the other two nations who the EU says have not taken action against the company.
Another three countries - the Czech Republic, Lithuania and Greece - are being hauled up for not even including the possibility of fining carmakers over potential violations.
On top of this, the European Commission has also called Germany and the UK to account for refusing to share details of breaches of EU emissions laws they discovered through their own investigations earlier this year.
Industry commissioner Elzbieta Bienkowska said in a statement: "National authorities across the EU must ensure that car manufacturers actually comply with the law."
Analysis, BBC business reporter Theo Leggett
The European Commission is showing distinct signs of frustration over the apparent unwillingness of European governments to take action against Volkswagen over its use of defeat devices - or even to keep it informed about their investigations.
In the United States, where the company was actually found to be cheating emissions tests, it has already reached a $15bn civil settlement with the authorities; it could face criminal fines as well. There has been nothing of the kind in Europe.
European law does state that carmakers which break the law should face penalties - but it's up to individual governments to enforce the rules and hand out punishments. It's worth remembering though that VW still claims that the software fitted to its cars was not actually illegal in Europe.
What the Commission really wants is the power to punish errant carmakers itself. That would, for example, prevent the governments of countries with powerful motor industries from being too soft on the manufacturers they would rather not upset.
There is already a proposal on the table to give it those powers, but it needs support from member states. This legal action - which could drag on for years - may just be a move to bring governments around to the Commission's way of thinking.
Fines and claims
Under EU law, member nations are responsible for overseeing whether cars meet standards or not.
Investigations in the UK, Germany, France and Italy have uncovered the use of cheat devices, but no action has been taken against VW, which employs 120,000 people in Germany with almost 500,000 employed elsewhere around the world.
The company has agreed to pay $15bn in a settlement with US authorities and owners of about 500,000 vehicles after the software cheat was exposed, but so far European nations have taken no such action against the company.
About 11 million cars worldwide have the software.
As well as fines, Volkswagen is facing €8.2bn ($9.1bn; £7bn) in damages claims from 1,400 German investors over its emissions scandal, a state court has said.
Australia also launched legal action against the carmaker and asset manager Blackrock and a group of institutional shareholders said they would sue VW for €2bn.
The claims relate to the drop in Volkswagen's share price after the scandal broke.
The VW group comprises 12 brands from seven European countries: Volkswagen passenger cars, Audi, Seat, Skoda, Bentley, Bugatti, Lamborghini, Porsche, Ducati, Volkswagen Commercial Vehicles, Scania and Man.