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An advert for an eco-friendly car overstated its environmental credentials, a watchdog has ruled.

The Saab BioPower advert showed parallel horseshoe prints and tyre marks and stated: "More horsepower. And a smaller carbon footprint.

"It's a fuel made from natural crops that reduces CO2 emissions by up to 70%.

"Bioethanol consumption does not significantly raise atmospheric levels of CO2 because the CO2 which is released when it is burned is counter-balanced by that which is removed from the atmosphere by photosynthesis when growing crops and trees for ethanol production."

But the Advertising Standards Agency (ASA) ruled: "We considered that readers were likely to infer that bioethanol did not add a significant amount of CO2 to the atmosphere.

"We considered that a 30% net addition to atmospheric CO2 was significant and therefore concluded that the claim was likely to mislead."

The ASA ruled that the national press advert broke rules on environmental claims, truthfulness and substantiation.

A spokesman for Saab said: "During the entire period that this advert ran there were only three complaints ... We do however take on board that we could expand on some of the detail in the small print and we have addressed this."

Other complaints, which said the claim that the "fuel reduces CO2 emissions by up to 70%" was misleading, were not upheld.