TWENTY years ago few motorists drove diesel cars.

Now a third of all cars on Britain's roads are fuelled with diesel.

Why? Because successive governments encouraged individuals, and the operators of company car fleets in particular, to switch to diesel via a variety of incentives because of the lower carbon dioxide emissions of such vehicles.

The moves were part of plans to meet EU climate change targets.

That was the carrot. Now drivers of diesels are about to feel the stick.

In London, mayor Boris Johnson wants to charge them £10 to enter the centre of the capital. The levy would be on top of the congestion charge.

If Mr Johnson gets his way it is thought many towns and cities will follow suit.

Why? Because while encouraging diesel cars cut carbon dioxide emissions it also massively increased nitrogen dioxide pollution, particularly in city centres.

The government and local authorities cannot have it both ways.

If the number of diesel cars on our roads increased because of incentives for motorists, then the same approach should be adopted to reduce them.

You cannot encourage something, realise it was a mistake, and then punish those who reacted positively to the encouragement.

We wish Mr Johnson all the worst with his latest brainwave.

And we hope Whitehall adopts a positive approach to correcting its mistake.