A new report finds that plug-in hybrid electric vehicles (PHEVs) are worse for the environment than previously thought. Analysis by the non-profit advocacy group Transport and Environment found that the real-world carbon dioxide emissions of PHEV models registered in 2023 are almost five times the official figures.

The vehicles use a combination of a combustion engine and electric motor, and are designed to be more economical and eco-friendly than traditional cars. Transport and Environment's report, published on Thursday, said data shows PHEVs emit 19% less CO2 than petrol and diesel cars. This is considerably different from laboratory tests which assumed they pollute 75% less, the report noted.

Sofía Navas Gohlke, a researcher at Transport and Environment and the report's co-author, said: "Real-world emissions are going up, while official emissions are going down.

"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 report said the gap in the estimates is mostly caused by "flawed assumptions" on the share of electric driving mode (which it describes as the 'utility factor') which leads to a "drastic underestimate of official PHEV emissions".

It said the utility factor "overestimated the electric driving share, assuming 84% over 2021-2023, whereas real-world data shows this to be just 27%".

The European Commission is making corrections to the utility factor ratio to better align it following the analysis, although there will still be a gap in the estimates, reports The Guardian.

The researchers at the Transport and Environment group analysed data from the onboard fuel consumption meters of 800,000 cars registered in Europe between 2021 and 2023.

Analysis also found that even when PHEVS are driving in electric mode, pollution levels are higher than previously estimated because the electric motors "have insufficient power and the combustion engine needs to kick in".

"The engine supplies power for almost one-third of the distance travelled in electric mode," the report said.

"This would mean an extra €250 [approximately £217] in petrol costs every year, as drivers don't expect to pay for fuel when driving in electric mode."

The UK Government is preparing to ban the sale of new fully petrol and diesel vehicles from the start of the next decade as part of Ed Miliband's push for net-zero emissions.

Sales of new PHEVs and hybrid vehicles will be allowed until 2035 under the plans.