After having a kind of renaissance during the first decade of the 2000s, diesel is now under attack again. The current generation of "clean" diesel vehicles might be much improved over the soot-machines of yore, but they are still a problem, especially when you pack a lot of them close together, as in Europe. Also, the facts as we know them have changed (like the Keynes quote: "When the facts change, I change my mind. What do you do, sir?"); We now know that diesel is more damaging to our health than we previously thought, with the World Health Organization officially recognizing it as causing cancer, and a recent study showing that it causes 6% of lung cancers in the US and UK.
The same thing might happen in the UK. Pollution levels have been over legal limits for nitrogen dioxide (NOx) since 2010 in 16 different cities and regions. The country's highest court has ordered the government to take immediate action to remedy the situation:
In a unanimous ruling, a panel of five judges, headed by the court's president Lord Neuberger, ordered "that the Government must prepare and consult on new air quality plans for submission to the European Commission... no later than December 31 2015". (source)
Something big will have to be done, because under existing plans, NOx limits set by the EU would not be met until 2030. Diesel is in the crosshairs because it is a big source of NOx, even with newer engines that produce 80% less than old models.
Air pollution in the UK is a serious matter. The official number of early deaths from it is estimated at 29,000 a year, which is more than obesity and alcohol combined.