Ford is developing a new Focus Econetic with sub-95g/km emissions and average economy of more than 80mpg.
Final figures will be confirmed later this year, but the provisional emissions and economy are lower than those of any other conventionally powered small family car.
The Focus Econetic will be available as a five-door hatch and estate from early in 2012, and is likely to cost from around 18,000. It will be exempt from the London Congestion Charge and attract zero road tax.
The car uses a new 104bhp version of the 1.6-litre TDCi diesel already available in the range.
No other performance details have been released by Ford, but the Econetic uses engine stop-start, regenerative braking, low-rolling-resistance tyres and revised gear ratios to achieve its impressive emissions and fuel economy figures.
The car's aerodynamics have also been worked on, with the addition of an active grille, which opens and closes shutters to allow for engine cooling with reduced drag.