Retail fuel margins are now 19 cents a litre, Mr Williams said, 12 cents more than the long-term average.
“Gasoline should actually be on sale for 153 pence per liter and diesel for 175 pence,” he said.
Mr Williams said independent retailers, which were traditionally much more expensive than their supermarket rivals, now often sell at lower prices.
Larger retailers tend to buy their gasoline further in advance, meaning changes in wholesale prices are filtering through to consumers a little more slowly, an industry source said.
Andrew Opie of the British Retail Consortium said: “Retailers understand the cost pressures motorists face and will do everything they can to provide the best value for money at their service stations, passing cost savings down the supply chain.”
An Asda spokesperson said: “Asda has consistently offered motorists the best value at the pumps. Since the beginning of July, our prices have fallen by 25 cents per liter on petrol and 18 cents per liter on diesel. In addition, today’s prices are 2 ppl per liter below the UK average for petrol and 3 ppl for diesel.”
Tesco, Sainsbury’s and Morrisons were approached for comment.
Earlier this year, the former prime minister said Boris Johnson declared war on gas stations who do not pass on the government’s fuel tax cuts, and undertake to name and shame those who refuse to lower their prices.
Johnson expressed anger that the 5 cents per liter fuel tax cut announced in March was not reflected in pump prices, prompting an investigation by the competition authority.