German car-maker Volkswagen is facing deepening scrutiny after being forced to admit millions of vehicles were fitted with software to cheat emissions tests.
Here are some key questions in relation to the emission test scandal.
How did the cars cheat the tests?
The US Environmental Protection Agency said VW cars were fitted with sophisticated software which detected when the vehicles were undergoing official emissions testing and switched the engines to a cleaner mode. It is a type of software known as a "defeat device".
Once on the road, the cars produced nitrogen oxide pollutants at up to 40 times the legal standard.
How many cars are involved?
Volkswagen has admitted that 11 million diesel vehicles worldwide were fitted with the software. The company was unable to give any details on how many are on UK roads.
What models are affected?
The US findings covered cars including the Audi A3 and VW's Jetta, Beetle, Golf and Passat models.
The Golf is the fourth biggest selling car in the UK this year at 45,597, according to the Society of Motor Manufacturers and Traders (SMMT).
VW said the issue relates to vehicles with EA 189 engines.
What is the impact on Volkwagen?
The company faces the cost of recalling millions of vehicles as well as a fine of up to 18 billion US dollars (£11.6 billion) in the US. Shares in the company were down 19% on Monday and have since fallen by a further 18%.
What is Volkswagen doing?
The company said it is "working at full speed to clarify irregularities". It has pledged to "eliminate" the difference in results between laboratory tests and road use.
What happens if cars in the UK are recalled?
Almost a million vehicles are called back to dealers in the UK for safety checks or work to rectify problems each year, according to the AA. The official recall scheme is overseen by the Driver and Vehicle Standards Agency.
The most common reasons for recalls are brakes, fuel, airbags, steering, seatbelts and fire risk.