The “excessive” margins on fuel by the UK’s big four supermarkets were further called into question today after the RAC found membership-only retailer Costco had slashed its petrol price to 147.7p a litre.

The RAC said the fuel price cut at membership-only retailer Costco “powerfully demonstrates how overpriced fuel is elsewhere”

The 8p reduction by Costco means unleaded now costs 12p less than the average price of a litre bought at a supermarket and 14p lower than the UK average.

Speaking today, the RAC added that the move by membership-only retailer Costco “powerfully demonstrates how overpriced fuel is elsewhere”, in particular at supermarkets -which have historically been a bastion of lower prices.

Earlier this week, it urged Asda, Morrisons, Sainsbury’s and Tesco to slash their prices in response to steadily falling wholesale costs – they’re currently enjoying margins of around 15p a litre on both petrol and diesel, leaving hard-pressed drivers having to fork out at an “unnecessarily high” price.

The average price of unleaded across the UK stands at 161.90p while diesel is 186.13p – at one of the four major supermarkets however, it is only slightly cheaper at 159.75p for petrol and 183.91p for diesel.

Diesel at Costco is currently on sale at its 19 forecourts for an average of 171.6p – nearly 14.5p less than the UK average and 12p cheaper than a litre at a supermarket. And at Costco Chester, unleaded is a remarkable 142.9p and diesel is 166.9p. Drivers filling up at Costco will generally save more than £6.50 a tank compared to a supermarket.

RAC fuel spokesman Simon Williams said: “The fact petrol and diesel can be sold for 12p per litre less than is currently being charged on average at the big four supermarkets is very concerning. Even accepting Costco operates on pretty thin margins, the supermarkets should be slashing their prices. On Tuesday this week we called on them to cut by 5p a litre but since then wholesale costs have fallen even further, so they should really be cutting by at least 7p.”

He went on to reason: “Given the huge gap between wholesale fuel prices and the averages charged by the supermarkets, it could be argued that drivers are being deprived the benefit of the Government’s 5p duty cut. This is unfair and we hope it is being duly noted by both the Government and the Competition and Markets Authority which is currently investigating UK fuel retailing.”

The RAC has recently reiterated advice to drivers to shop around on fuel – many independent retailers are now cheaper than the supermarkets.

“Other than trying to fill up somewhere cheaper, there’s very little drivers can do to avoid shelling out more than they should have to in the cost-of-living crisis,” Williams summed up.