Here are some of the key questions on diesel:
I thought drivers were urged to buy diesel cars?
Motorists were encouraged to switch away from petrol under Tony Blair's government as diesel cars generally emit less CO2 because they are more fuel efficient.
Then-chancellor Gordon Brown reduced duty on low-sulphur fuel in 2001, which contributed to an increase in diesel car registrations.
When did it start to go wrong?
Volkswagen sparked outrage in September 2015 when it was found to have fitted defeat device software to 11 million diesel vehicles worldwide.
This allowed the manufacturer to cheat tests for nitrogen oxide pollutants and sparked a rise in concern about air pollution.
How significant is the fall in demand for diesel?
UK new car diesel sales dropped by 17% last year with a market share of just 42%, compared with 48% in 2016.
Its performance was even worse in December, with year-on-year sales down 31%.
What action has the Government taken?
From April every new diesel car will be liable to a one-band increase in the first-year vehicle excise duty rate.
Plans have also been unveiled to ban the sale of all conventional new diesel and petrol cars by 2040.
What about local authorities?
Councils are being asked to come up with ways of raising air quality in towns and cities, ranging from improving public transport and changing road layouts, to charging zones for polluting vehicles.
What does the motor industry say?
The Society of Motor Manufacturers and Traders claims "confusing anti-diesel messages" are reducing fleet renewal, which it says is the best way to tackle air pollution.
Many drivers are keeping hold of their older, more polluting cars because of uncertainty over future regulations.
What does the future hold for diesel?
The SMMT believes the decline in diesel's market share will slow this year, reaching around 40%.
Will electric cars take over?
Alternatively-fuelled vehicles, such as hybrids and pure electric, achieved a record market share of 4.7% in 2017.
But it is still far behind petrol and diesel cars.
Motoring groups say the quality of the charging network, range of batteries and price of electric cars all need to improve to persuade more motorists to make the switch.