A new "greener" petrol is actually less efficient and will cost drivers more than an alternative already on the market, research by a consumer car magazine suggests.
The introduction of new E10 fuel is due to take place this year in line with EU regulations and as part of the Government's commitment to reducing greenhouse gas emissions.
E10 - containing 10% bio-ethanol - meets the EU's Renewable Energy Directive, which says that 10% of road transport energy must be from renewable sources by 2020.
But savvy motorists who are already doing everything they can to cut costs - by buying the most efficient cars and finding the bestcar insurance deals - may be far from pleased at the prospect of paying more for their fuel.
What Car? carried out the first real-world tests on E10 and discovered it is less efficient than the current E5 blend of fuel across every type of engine they tested.
With cars needing more of the new fuel and drivers having to spend more regularly to fill their tanks, What Car? editor-in-chief Chas Hallett has urged the Government to carry out comprehensive, UK-focused testing in order to better understand the financial impact.
"To lead consumers into E10 without fully communicating the significant impact on fuel economy, particularly for drivers least able to absorb the extra costs, is irresponsible," he said.
He was referring there to the finding that smaller cars, which are often bought by drivers with less spare cash, are worst affected by the higher ethanol content.
RAC technical director David Bizley said: "The European Directive is clearly well-intentioned, but it is both surprising and worrying that What Car has found in tests that carbon dioxide emissions actually increased and fuel economy fell by as much as 10% on vehicles using E10.
"If these results were borne out by larger scale testing, it would raise questions as to whether there is sufficient environmental benefits to justify the switch to higher biofuel content petrol.
"Less well-off motorists are still finding it very difficult to find the money to pay for the fuel for even the most essential journeys and the Government has recognised this by freezing fuel duty to the end of this parliament. If motorists are required to use a less efficient fuel then Government should be considering reducing the fuel duty on E10 to offset any reduction in fuel economy.
"Other concerns for drivers of vehicles made prior to 2002 using E10 have also been reported. These are associated with possible damage that can be caused by bioethanol's corrosive properties which can lead to damaged seals, plastics and metals. There have also been reports that E10 is a less stable fuel and that this can make it more difficult to start a vehicle that has not been driven for an extended period."
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