Plug-in hybrid cars burn significantly more fuel than official tests record, according to research that suggests pollution from the vehicles could be much worse than advertised. Tests of plug-in hybrid electric vehicles (PHEVs) by Which?, the UK consumer group, found that some popular cars achieved as little as a third of the fuel economy advertised in official tests. Cars produced by BMW, Mercedes-Benz, Mitsubishi, Volkswagen and Volvo were among 22 tested that on average burned 61% more fuel than in official tests. The worst performer was BMW’s X5 plug-in hybrid SUV. It achieved 188.3 miles per gallon in official tests, versus only 52.8 miles for Which?. That 72% efficiency difference could potentially add more than £650 a year in petrol costs, the group said. The extra fuel burned would result in higher carbon dioxide emissions, as well as increasing running costs for owners. The average extra cost was £462, Which? estimated. Plug-in hybrids combine an internal combustion engine with an externally rechargeable battery, in the hope of lowering emissions of CO2 and other pollutants while also giving the range and refuelling ease of fossil fuels. However, the environmental benefits of PHEVs – often fitted in bulky SUVs – are controversial. The car industry argues that they are a crucial stepping stone for some consumers to move to fully electric cars, but some environmental campaigners argue that the cars can actually be less green than equivalents without a battery if they are not regularly recharged. All cars sold in Europe must, by law, be tested according to the worldwide harmonised light vehicle test procedure (WLTP). Which? argues they do not accurately reflect real-world driving conditions, and so tests cars over long ranges while using air conditioning and the radio, as well as running PHEVs for portions of its tests with an empty battery. Greg Archer, the UK director at Transport & Environment, a campaign group, said official fuel economy and emissions test results were “a cruel distortion of reality”. He said: “The engine in most PHEVs turns on when the car accelerates hard, or the driver switches on the heating or air conditioning. Most PHEVs have such a small battery the car can only drive around 30 miles with zero emissions and can’t be fast charged. Owners wanting to reduce their running costs, or choose a green car, should pick a battery electric model.” PHEVs accounted for 58% of all rechargeable cars sold in the biggest 18 western European markets in January, according to data from Matthias Schmidt, a Berlin-based electric car analyst. Plug-in hybrids will remain popular over traditional internal combustion cars because of tax and subsidy advantages, he said. The cars are also a key part of carmakers’s strategies to cut their average carbon emissions and avoid steep fines. Volkswagen and Jaguar Land Rover, the UK’s largest automotive employer, are among the carmakers who have been forced to pay out millions of euros for breaching EU targets in 2020. Which? put its findings to all of the carmakers whose products were tested. Those who responded highlighted that PHEVs can lower emissions if recharged as recommended. Mike Hawes, the chief executive of the Society for Motor Manufacturers and Traders, the UK industry lobby group, said carmakers were not allowed to publish emissions data other than WLTP. He added: “The WLTP tests consistently demonstrate that plug-in hybrids offer comparable range to pure petrol or diesel equivalents but deliver substantial emission reductions, with zero-emission range typically 25-40 miles, which is more than ample, given that 94% of UK car journeys are less than 25 miles.”