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Plug-in hybrid electric vehicles (PHEVs) pump out nearly five times more planet-heating pollution than official figures show, a report has found.

The cars, which can run on electric batteries as well as combustion engines, have been promoted by European carmakers as a way to cover long distances in a single drive – unlike fully electric cars – while still reducing emissions.

Data shows PHEVs emit just 19% less CO₂ than petrol and diesel cars, an analysis by the non-profit advocacy group Transport and Environment found on Thursday. Under laboratory tests, they were assumed to be 75% less polluting.

The researchers analysed data from the onboard fuel consumption meters of 800,000 cars registered in Europe between 2021 and 2023. They found real-world carbon dioxide emissions from PHEVs in 2023 were 4.9 times greater than those from standardised laboratory tests, having risen from being 3.5 times greater in 2021.

“Real-world emissions are going up, while official emissions are going down,” said Sofía Navas Gohlke, a researcher at Transport and Environment and the co-author of the report. “This is the gap that is getting worse and it is a real problem. As a result, PHEVs pollute almost as much as petrol cars.”

The researchers attributed most of the gap to overestimates of the “utility factor” – the ratio of miles travelled in electric mode to the total miles travelled – finding that 27% of driving was done in electric mode even though official estimates assumed 84%. The European Commission has announced two corrections to the utility factor ratio that will narrow the gap but not close it entirely, according to the analysis.

Even when the cars were driven in electric mode, the analysis found that levels of pollution were well above official estimates. The researchers said this was because electric motors were not strong enough to operate alone, with their engines burning fossil fuels for almost one-third of the distance travelled in electric mode.

Patrick Plötz, head of energy economics at the Fraunhofer Institute for Systems and Innovation Research, who was not involved in the study, said it was a “very useful contribution” after years in which parts of the automotive industry argued there was too little data to accurately assess real-world emissions.

“The results demonstrate, beyond any doubt, that the gap between official and real-world PHEV fuel consumption and CO₂ emissions is much, much larger than for gasoline or diesel cars,” said Plötz, who has published research on the topic. “Any policy changes with respect to PHEVs should be made with utmost care and in the light of that data.”

Hybrid cars have been drawn back into the political debate as carmakers have pressed the EU to weaken CO₂ targets. A ban on new combustion engine cars in 2035 has been subject to heavy lobbying from the automotive industry and opposition from member states with large car industries.

“There must not be a drastic cut in 2035,” the German chancellor, Friedrich Merz, said after a summit last week with the country’s struggling automobile industry, promising to do “everything in [his] power” to achieve that. Other senior German politicians have floated plug-in hybrids as one example of possible “flexibilities” they could introduce to the legislation.

The researchers calculated that the underestimate of PHEV emissions had let four major carmaker groups avoid more than €5bn (£4.3bn) in fines between 2021 and 2023, by making it artificially easier to comply with the EU’s fleet-average CO₂ targets. They added that drivers of PHEVs would also be paying about €500 more a year in running costs than would be assumed under laboratory tests.

“The bold claims that manufacturers like to make about their plug-in hybrid vehicles are clearly way off the mark,” said Colin Walker, a transport analyst at the Energy and Climate Intelligence Unit.

“Consumers are being duped into believing that in buying a PHEV, they are helping the environment and saving money,” he said. “In reality, PHEVs are little better than regular petrol and diesel cars when it comes to the fuel they consume, the CO₂ they produce and the money they cost to run.”