Fuel prices have fallen for the second consecutive month but are still overpriced by up to 8p a litre.

The RAC says both petrol and diesel are still too expensive on forecourts in England, Wales and Scotland

The average price of petrol across the UK fell by 3p a litre last month to just under 145p, RAC analysis reveals. Diesel dropped nearly 4p, from 153.74p to under 150p (150.13p) for the first time since early February.

Prices have fallen despite a steady increase in the cost of oil over the month, which has caused the wholesale price of fuel to go up slightly,.

But the RAC says both petrol and diesel are still too expensive on forecourts in England, Wales and Scotland.

It points to prices in Northern Ireland where a litre of petrol sells for an average of just 140.5p – 4.5p cheaper than the UK average. Diesel is 142p – 8p less than the UK-wide price.

Fuel prices at the big four supermarkets – Asda, Morrisons, Sainsbury’s and Tesco – fell by an average of 4p for both petrol and diesel in June, which meant the average cost of a litre dropped to 142p for petrol and 147p for diesel.

Further analysis reveals wide variations among the supermarkets between their cheapest and most expensive sites. For petrol, these range from just 6p across Tesco sites, to 10p at Morrisons, 13p at Sainsbury’s and 35p at Asda – the latter is affected by the fact the retailer now has 665 sites, many of which are non-supermarket forecourts since it acquired the 350-strong Euro Garages portfolio.

For diesel, the gap is bigger. Tesco still has the smallest difference between its high and low prices at 9p, whereas at Morrisons it’s 15p and at Sainsbury’s it’s 16p, while at Asda stands at 38p.

RAC analysis of data provided to the Competitions and Market Authority (CMA) watchdog reveals that Shell-owned forecourts have the most expensive petrol in the UK at 149p a litre – 4p more than the UK average and 8.5p more than in Northern Ireland.

BP-operated forecourts are not far behind with an average of 146.5p. Shell also has the highest average for diesel at 155p – 5p more than the UK average and 13p more than the average in Northern Ireland. BP’s average is 152p across the 287 sites it owns and runs.

RAC head of policy Simon Williams said: “We remain baffled how the very same fuel can be sold for such vastly different prices by the biggest retailers, whether they’re run by supermarkets or the world’s largest oil companies.

“It’s also the case that while oil has increased from under $80 at the start of June to the mid-$80s by the end, wholesale costs are still low enough to merit cheaper prices at the pumps. Looking at the fairer average prices charged in Northern Ireland, petrol should be 4.5p lower across England, Scotland and Wales and diesel should be a whole 8p less.

“We will continue to highlight this disparity, along with the massive differences between major retailers’ high and low prices, to the new government and the Competition and Markets Authority with a view to them being addressed by the new Pumpwatch scheme when it is up and running.”