Petrol and diesel prices on supermarket forecourts have risen every day since the end of March, according to data from RAC Fuel Watch.

At the end of May, pump prices at the big four supermarkets reached averages of 124.74p for petrol and 127.69p for diesel. This represents an increase of 8p per litre since the same period in March, from 119.78p and 122.54p for petrol and diesel respectively.

Across the UK average prices increased by 8.5p a litre. The increases are attributed to oil prices rising to above $80, as well as weak pound-to-dollar exchange rates negatively affecting the cost of fuel on the wholesale fuel market.

Simon Williams, RAC fuel spokesman, said: “This is the worst series of consecutive daily supermarket price rises we have seen since we began monitoring this three and a half years ago.

“Sadly, RAC Fuel Watch data shows [supermarkets] are far faster at passing on rises in the wholesale price than they are falls,” he said.

“Supermarkets are highly influential in the price charged at pumps across the UK as, despite operating under 18% of all forecourts, they sell 45% of all fuel sold.

“While other fuel retailers don’t buy as frequently they will often match supermarket price rises which means they increase as the supermarkets do.”

“Unfortunately, the higher oil price has come at a time when the pound has weakened considerably against the dollar,” continued Williams. “This causes pain at the pumps due to the fact that, similar to oil, fuel is traded in dollars so the lower the exchange rate, the worse the price gets for UK motorists.”

The last time oil prices reached $100 per barrel, forecourt prices for petrol and diesel were at around 129p and 133p a litre, with pound sterling at $1.67 (worth around 20% more than presently). If today’s oil prices increased to $90 a barrel with the pound staying at $1.33, average petrol pump prices could reach 137p a litre.

“This is sadly a very clear demonstration of how important the strength of sterling is in the price motorists pay for fuel,” Williams added.

“The outlook for fuel prices in the weeks ahead is not good with another penny a litre expected to be added to the current average prices of petrol and diesel.

“All we can do is hope the international forces which have caused the oil price to rise ease and take the heat out of prices on the forecourt.”