He lied

The admission by, T. King, the government’s chief scientist during the New Labour era, that he was well aware that diesel is ‘dirtier’ than petrol (as far as emissions go) and kept quiet about it confirms what most of us have suspected for a long time. If it’s anything to do with the government, you can’t trust the politicians or their hirelings to tell the truth if a lie suits the politics of the moment better.