The Toyota Prius and Honda Civic Hybrid are currently top when it comes to waving their green credentials, but in terms of CO2 emissions, this time next year the top two eco-friendly cars will be produced by Smart.

Both products will be based around the all-new ForTwo city car that goes on sale next month. The first will be badged the MHD, for Micro Hybrid Drive, and will arrive in showrooms early next year with a GBP150 price premium over the standard version. It features stop-start technology thanks to belt-driven starter generator that cuts the engine when the car comes to halt at traffic lights, junctions and the like.

MHD will be available as a coupé or cabrio and in all three trim levels, but only with a 71bhp engine. Fuel economy rises from 60.1 to 65.7mpg, and there's a reduction in CO2 emissions from 112 to 103g/km.

That undercuts the Prius and Civic at 104 and 109g/km respectively.

But the ace up Smart's sleeve for the UK is a ForTwo 800cc diesel with impressive figures of 85.6mpg and 88g/km of CO2.

However, there is a downside - it's not exactly what you'd call fast. The two-seater doesn't hit 60mph for 19.8 seconds and top speed is only 84mph. Oh yes… and it's left-hand drive.

A spokeswoman said: "We're not expecting a large number of orders but because of the low CO2 emissions it's worth us offering. Like all new Smarts, it will be exempt from the London congestion charge when the emissions-based tolling proposal takes effect."

She said she believed the diesel would appeal to urban-based commuters with short journeys who didn't need particularly quick acceleration. "We will be numbers one and two in the country on emissions, which is a great story for Smart," she added.