Theresa May is being urged to investigate fuel pump pricing as the fuel supply chain is accused of holding back wholesale changes amounting to at least £6bn in the last 12 months.

Wholesale diesel and petrol prices both dropped 4p, yet the pump price fell by only 1p.

The call comes from the FairFuelUK campaign, which says that wholesale and retail data collected in the last 12 months puts paid to government claims that pump fill-up costs reflect oil price changes within seven weeks.

Announced as supermarkets slash fuel prices by up to 2p per litre, the campaign says that in the last seven weeks, wholesale diesel and petrol prices both dropped 4p yet the pump price fell by only 1p.

Referring to “perennial fleecing” of drivers and small businesses, Fairfuel UK added that in the last six months, wholesale diesel increased 4p yet the pump price went up 8p, while wholesale petrol increased 4p yet the retail price went up 7p.

Charlie Elphicke, Conservative Member of Parliament for Dover & Deal and chair of FairFuel All-Party Parliamentary Group, said: “This is strong evidence that drivers are being ripped off by greedy big oil companies. Pump prices fall like a feather but jump like a rocket when the oil price changes. A powerful PumpWatch watchdog would help millions of drivers get a fairer deal.”