Experts at Which? said electric cars are a lot more expensive to buy, though they are generally cheaper to run as they plug in for their power from the domestic mains.
However, the amount of carbon dioxide created to generate the electricity powering an electric car can be just as great as that created by the internal combustion engine, reports the Daily Mail.
The main difference is that while a conventional car's emissions come out of the vehicle's exhaust pipe, those created by an electric car are generated at the power station, which supplies the electricity.
The findings come as the first ever electric car to pass the European crash test was announced - the Mitsubishi i-MiEVsuper-mini - getting four stars out of a maximum five.
Experts at Which? compared the carbon dioxide created by charging electric cars with that emitted by the most efficient diesel models and concluded that 'sometimes there's not a great deal of difference'.
And the gap is narrowing as 'conventional' cars up their game to cut emissions.
"The common manufacturer claim that electric cars produce 'zero emissions' ignores the fact that most drivers use a conventional electricity supply to charge them, which has a carbon cost from burning fossil fuels," the Which? report noted.
To test its theory, Which? looked at three of the first electric cars destined to hit the UK market and put them up against three 'efficient' conventional rivals.
The experts found, for example, that the electric Smart Fortwo, expected to cost around 21,000 pounds, creates an 'equivalent' of 84 gm of CO2 per km driven, whereas the 9,540-pounds diesel Smart Fortwo emits 103 gm.
It also compared the Nissan Leaf, the 23,990-pounds electric car, with Volkswagen's diesel Golf 1.6 TDi Bluemotion costing 16,830 pounds.
The electric power generated to drive the Leaf is equivalent to CO2 emissions of 81g/km. By contrast, the diesel Golf has CO2 emissions of 108g/km.
Two 'super-minis' were also compared. Which? found that the power generated to power the 24,045-pounds Mitsubishi i-MiEV electric car was equivalent to 68g/km.
Then similarly sized Suzuki Splash costing 10,410 pounds with a 1.3litre diesel engine has CO2 emissions of 131g/km.o measure the carbon dioxide created by charging an electric car, Which? followed the advice of the Carbon Trust which states that 544gm of CO2 are emitted per kilowatt hour of electricity used.
However, electric cars are much 'greener' than diesel cars when it comes to localised emissions, as they don't emit toxic chemicals that degrade air quality. This is especially significant in cities, where the uptake of electric cars is predicted to be highest, said Which?