Although fuel prices have fallen in recent months, the cost difference between fuels has grown dramatically.
During August, the average gap between petrol and diesel was 1.24 pence per litre, but by the end of last week that had grown to 5.07p.
The difference has lead to fears from the AA Motoring Trust that fleets and diesel drivers could be subsidising their petrol-using counterparts, as it was revealed the average wholesale difference in price has remained at just 3.4 pence. Usually, the wholesale price is a trade secret but it was leaked to the media.
A spokesman for the trust said: ‘Retailers have the right to charge whatever they like for their products. They are entitled to manipulate prices for whatever reason, perhaps promotions or to influence customer flow.
‘However, without freely-available wholesale prices, consumers, business and fleet managers have no way of knowing whether the price they pay for their diesel is good value, or whether it is cheaper or more expensive.
‘Call it a rip-off or call it natural market forces, the facts of the matter are that businesses with diesel cars on their fleet are probably paying above the odds for their fuel. This sort of price manipulation goes unnoticed because car fuel wholesale prices are hidden.’
Mike Waters, head of market analysis for fleet and fuel management firm Arval, said the difference was due to supply and demand.
‘Demand for diesel on a Europe -wide basis, particularly in the UK, has increased at a time when there is a lack of diesel refining capacity globally,’ he said.
Waters said at the current prices, a fleet of 100 vehicles, each travelling 20,000 miles a year, will incur a fuel cost of £288,000 for a petrol fleet and £200,200 for a diesel fleet, because of better fuel economy in diesel vehicles.
The Petroleum Industry Association was unavailable for comment.