It turns out tens of thousands of plug-in hybrids (PHEVs) in corporate fleets may have never been... 'plugged-in'.

According to the BBC, numerous cars in corporate fleets are averaging just 40 miles per gallon, which is a figure closer to a general combustion engined car without hybrid assistance – they should be achieving 130 mpg.

2017 Toyota Prius

The Miles Consultancy, which advises 300 companies on fuel management showed that mileage records from 1,500 cars showed an average real-world mpg of 39.27, instead of an average manufacturer advertised mpg figure of 129.68.

So why are so many hybrids not being used as intended? Well, it all comes down to the government plug-in grant that was introduced in 2011, which has since made the UK the biggest PHEV market.

The grant gives buyers up to £4,500 off new cars, and The Miles Consultancy says that company's were just using the grant to get money off new vehicles to fill their fleets.

Hybrid car plugged into charging station on street

"There are some examples where employees aren't even charging these vehicles up," Paul Hollick, The Miles Consultancy's managing director, told the BBC.

"The charge cables are still in the boot, in a cellophane wrapper, while the company and the employee are going in and out of petrol stations, paying for all of this additional fuel," he added, calling the whole scenario "ridiculous"

The majority of the country's PHEV sales have been to fleets, including 70 percent of the 37,000 sold so far this year, and the British Vehicle Rental and Leasing Association (BVRLA) has admitted that higher taxes on diesel vehicles had led to many fleets opting for hybrids, even if they don't intend to use them properly.

2017 Volkswagen e-Golf

"We unfortunately have got a situation where a poorly designed tax regime is driving some poor behaviours," said ​the BVRLA's communications director Toby Poston. "We have got some situations where company drivers are choosing the vehicle based on their tax liability, rather than having the right vehicle for the right job."

The plug-in car grant is set to be scrapped, with the government's attention turning to all-electric vehicles instead, but the move hasn't been warmly received.

"We need policies that encourage consumers and businesses to invest in the cleanest cars, however they are powered," the chief executive of the Society of Motor Manufacturers and Traders, Mike Hawes.

Electric vehicle charging station sign in parking bay

Gallery: 2017 Volkswagen E-Golf First Drive