Petrol retailers have been warned by the UK competition regulator about the speed with which prices have come down in recent weeks and how much real competition there is in the marketplace.
Having already found that motorists were paying over the odds last year, the Competition and Markets Authority (CMA) said it was concerned that the spread or difference between pump prices and benchmarked wholesale prices had widened.
"During September and October, the CMA observed significant increases in retail spread for both petrol and diesel," the watchdog said.
"In both cases, the retail spread at the end of October was significantly higher than the long-term average of 5-10ppl (pence per litre).
"While it is expected that the retail spread will increase and decrease in response to volatility in wholesale prices, over time pump prices should track wholesale prices if retail competition is effective.
"If retail spreads were to remain at these levels for much longer, this would cause concern about the intensity of retail competition in the sector."
Since May, prices for petrol have risen by 11p and by 13p for diesel, which motoring groups say reflects margin padding by retailers though the CMA said it needed more data to be certain.
Sarah Cardell, chief executive, commented: "The underlying data shows a mixed picture in terms of what is driving this.
"Over the summer we saw rising wholesale costs, but more recent trends give cause for concern that competition is still not working well in this market to hold down pump prices.
"We will be monitoring and reporting further on this in our next update."
Luke Bosdet from motoring group the AA was more forthright: “Old habits die hard in the road fuel trade," he said.
"Failure to pass on the full savings from lower wholesale costs to hard-pressed motorists, their families and businesses is unacceptable in a cost-of-living crisis.
"The feedback from drivers is that they want more transparency."