Tens of thousands of plug-in hybrids (PHEVs) bought with generous government grants may be burning as much fuel as combustion-engine cars.
Data compiled for the BBC suggests that such vehicles in corporate fleets averaged just 40 miles per gallon (mpg), when they could have done 130.
Many drivers may never have unwrapped their charging cables, The Miles Consultancy said.
Subsidies for new PHEVs were recently scrapped, after seven years.
The plug-in grant was introduced in 2011, gifting buyers up to £4,500 off new cars.
The incentive helped the UK become the biggest market for PHEVs in Europe.
The majority of the tens of thousands of eligible vehicles sold were bought by company fleets, including more than 70% of the 37,000 plug-in hybrids sold so far in 2018.
But data from The Miles Consultancy, a Cheshire firm which advises 300 blue-chip companies on fuel management, reveals that many businesses simply used the grant to save on buying regular cars.
Mileage records from 1,500 models, including Audi, BMW, Mercedes and Volvo vehicles, showed an average real-world mpg of 39.27, against an average manufacturer advertised mpg of 129.68.
Figures for 2,432 hybrids – including non plug-in varieties – showed an average real-world mpg of 49.06, still vastly lower than the potential range.