Volkswagen has said as many as 11 million of its diesel cars could be affected by the emissions testing scandal.

• What is the problem?

US authorities say the company deliberately circumvented clean air rules on diesel cars by using software which was triggered when the cars were undergoing exhaust tests.

The "defeat devices" meant that true emissions levels were hidden, it is claimed.

• Which models are affected?

The Jetta, Beetle, Audi A3 and Golf manufactured from 2009-2015 are affected, as well as the 2014-15 Passat models.

Volkswagen said the problem related to vehicles with Type EA 189 engines.

As a result, Volkswagen has stopped the sale of 2015 models and is prohibited from selling 2016 models until the programme is fixed.

It has been ordered to recall 482,000 cars in the US.

It is not clear how many vehicles are affected in each country.

• Are the affected cars safe to drive?

The cars are safe to drive, and Volkswagen has said it will fix the cars for free as soon as it develops a way to solve the problem.

• What is Volkswagen doing about it?

The company has said it will set aside €6.5bn as part of its efforts to win back trust and cover the costs of fixing the problem, and vowed to communicate "transparently" in the future.

CEO Martin Winterkorn has apologised, promised an internal investigation and acknowledged the firm had "broken the trust of our customers and the public".

• Is Volkswagen the only car maker at fault?

At the moment. However, other manufacturers are under pressure to clarify whether they have ever used similar software in their vehicles.

Shares in the likes of BMW, Peugeot and Renault dropped on Tuesday despite no evidence of involvement.

The US Environmental Protection Agency has said it will screen other carmakers for the "defeat devices".

France has called for a Europe-wide probe into Volkswagen's practices - and into those of French manufacturers.