Diesel-graphicLondon Mayor Boris Johnson’s plan to charge dirtier diesels to enter the Congestion Charge zone risks leaving motorists feeling “betrayed and misled”, says AA President Edmund King.

The ‘dash for diesel’, he explained, was encouraged because lower CO2 emissions chimed with the “near hysteria about carbon dioxide, and yet nobody looked at the bigger picture”.

Now, drivers who thought they were doing the right thing “are being told that it has serious health implications [due to higher nitrogen oxide emissions – commonly called NOx]. They are being made to feel guilty for something that they were encouraged to do.”

King, speaking to the Telegraph, said the ‘Boris diesel tax’ was the first move by authorities to tackle dangerous levels of inner city NOx – and where London leads, others will follow. As many as 18 other cities could quickly replicate the London Mayor’s proposed new tax.

And that’s not all. King also expects the Treasury itself to “slap extra tax penalties on diesel vehicles”. The government would thus be taxing vehicles it initially encouraged with tax breaks.

King’s words could have a significant impact on motorists’ views: over the past decade, diesel cars have swelled from 1.6 million vehicles to more than 11 million. In other words, 1 in 3 cars on Britain’s roads is diesel-powered (and they now account for 1 in 2 new car sales).

Euro 6: the diesel engine NOx fix

Newer diesels will be exempt from any taxes. Emissions legislation has led to extraordinary reductions in diesel engine emissions since the early 1990s, and a modern Euro 5 diesel emits many magnitudes fewer emissions than older motors. The sticking point has long been NOx emissions, but they are finally being virtually eradicated from tailpipe output with the imminent Euro 6 emissions standard.

This is mandatory for all new-to-market cars sold from September 2014, and every single new car sold from September 2015. It takes diesel NOx emissions down to almost the same level as petrol cars – a 70% reduction on the current Euro 5 legislation – and for the first time ensures virtual parity between petrol and diesel in key exhaust emissions.

It is Euro 6 cars that will escape the Boris diesel tax from 2020.

Even so, for all other cars, a £10 additional charge will be payable. Today, most of the 11 million diesel cars on Britain’s roads would be required to pay it; come 2020, there will be fewer of them in use, and even fewer regularly travelling into central London, but the message of this potential new legislation is clear.

CO2 incentives have changed the UK new car landscape enormously: the Boris diesel tax is the first sign that this may be about to change. And the more bodies that follow suit and mimic it, the more motorists will indeed feel ‘betrayed and misled’. As there are now so many diesel cars on Britain’s roads, the ramifications could be significant.