Motorists are complaining that E10 petrol is much less efficient than traditional E5 fuel, costing them a fortune to fill up their cars.
The news comes as many drivers report not being able to find petrol near them due to shortages at forecourts.
E10 is a greener type of fuel that replaced E5 as the standard for unleaded petrol from last September - and now even Formula 1 racing cars run on it.
Most garages in England, Scotland and Wales now sell E10 as their basic '95-octane' grade unleaded, with the option of E5.
But drivers who have been using E10 have complained that they’re getting fewer miles to the gallon - meaning they’re having to fill up more frequently.
When E10 was first rolled out, the government said it was 1% less efficient than E5, but that it was greener and cheaper.
E10 contains 10% bioethanol to 90% petrol, compared to 5% bioethanol in E5.
But drivers have taken to social media to say their cars have fuel efficiency far worse than the government's 1% figure.
Does E10 offer less value for money? Let us know in the comments below
One motorist said E10 was an astonishing 35% less efficient than E5 when running his car.
The driver said: "My car was giving me 350-380 miles per tank on petrol but since this E10 ethanol watered down garbage came in I'm now at 250-280 miles."
A second car owner said his household Honda Jazz got 40 miles per gallon (mpg) with E5 but only 35mpg with E10 - a 14.2% difference.
Yet another driver said his car was 16% less efficient with E10 petrol.
The motorist compared how much it cost to run his 2004 Peugeot 206 with E10 compared to E5.
He found he got 46.7mpg with E5 compared to 40.1mpg with E10 - a difference of 16.4%.
Even factoring in the fact E5 costs more than E10, the driver said the lower efficiency of E10 means he would save £72.95 over 10,000 miles if fuel prices were to stay the same.
So despite the higher cost of E5, over 10k miles (assuming price stay the same), it works out £72.95 saving running the car on the more expensive E5 petrol than the same mileage using E10
— Andrew (@TK42138)
While E10 petrol is clearly less efficient than other variants, drivers are still hotly debating the extent of any engine harm - with some saying it is totally fine.
Before garages began stocking E10, the government said up to 1million pre-2011 cars and motorbikes should not use it in case it caused engine damage.
But some drivers and garages think the fuel causes engine issues to newer cars too.
But major garages like Halfords, ATS Euromaster and Qwik Fit said their mechanics have not seen noticeable numbers of cars with engine damage from E10 fuel.
The E10 issue is compounding the pain drivers are feeling with soaring fuel prices.
The average litre of petrol costs 161.7p a litre, compared to 125.4p a year ago, according to government figures today.
Diesel is now 176.2p a litre, and it was 129.3p 12 months ago.
Current fuel prices have dipped slightly from the all-time highs seen a month ago, but are still extortionate as households contend with the cost of living crisis affecting energy bills, council tax, food and more.
