Drivers are being urged to think twice before travelling to their local supermarket after a veteran mechanic issued an urgent fuel warning.
There are estimated to be around 1,500 supermarket petrol stations around the UK, with millions of drivers visiting them every week to fill up and seek out the cheapest costs.
At present, the average cost of a litre of unleaded petrol in the UK is 143.25p per litre, while diesel drivers are expected to fork out 148.14p.
The data from RAC Fuel Watch shows that petrol and diesel prices are expected to fall further, having dropped by around 2p per litre for each fuel type over the last few weeks.
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Shell has some of the highest average fuel prices in the UK
PA.However, Reaz, a professional mechanic of 17 years and garage owner, has urged motorists to "avoid supermarket petrol at all costs".
He noted how fuel is one of the most overlooked maintenance tips given how readily available supermarket filling stations are around the UK.
Reaz claimed that supermarket fuels often lack the "necessary detergents" required to keep the fuel system clean, while calling on drivers to alternate between premium and standard types of fuel.
He said these measures would prevent premature system failures. He added: "It's a simple practice that can extend the life of your vehicle's engine."
Tesco and Morrisons are the joint cheapest major supermarkets, charging just 139.7p for petrol on average. In comparison, Sainsbury's charges 140.4 per litre, while Asda is the most expensive at 141.4p.
It also reveals that Tesco has the cheapest diesel prices of any of the big four supermarkets with an average cost of 144.5p per litre.
This is just 0.2p cheaper than Morrisons on average, while Tesco shoppers can save 0.8p per litre more than Sainsbury's and 1.6p more than Asda.
There has been a push for competition among major retailers in recent months, evidenced by Essar charging an average of just 136.9p for petrol.
At the other end of the price spectrum, Esso, BP and Shell are all charging the most, at 143p, 144.5p and 145.4p respectively.
The RAC Fuel Watch data also shows how some Shell forecourts charge as much as 168.9p per litre for petrol, a significant 35.1p difference compared to its cheapest filling station (133.8p).
The same trends can be seen with diesel prices, too. Essar has the cheapest costs at 141.9p per litre, while Shell is the only major retailer listed that charges more than £1.50 per litre.
Drivers are also consistently urged to avoid visiting petrol stations at motorway service stations as these have some of the most inflated prices across the country, given that they have a captive audience.
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Motorway service stations can be more than 23p more expensive per litre
Scrap Car ComparisonThe average price of unleaded petrol and diesel is more than 23p more expensive at motorway service stations compared to the UK average.