Fuel retailers overcharged motorists when the coronavirus lockdown was introduced by failing to fully pass on savings, according to analysis.
Consumer group Which? found that average profit margins rose from 10p per litre to 18p in the weeks after movement restrictions were introduced in March.
It said that pump prices dropped below £1 per litre at some supermarkets as the price of oil plummeted but drivers were still overpaying as fuel companies pocketed a chunk of the savings.
The organisation acknowledged that smaller independent petrol stations may have needed to raise profit margins to survive the pandemic, but claimed larger forecourt groups will have saved millions during the crisis.
There are no rules on the mark-up that retailers can use when selling fuel.
Which? editor Harry Rose said: “While there may have been fair cause for some fuel sellers to increase retail margins in order to survive lockdown, there really is no excuse for some larger retailers to be keeping savings for themselves during the pandemic.
“For customers to be charged fairly at the pumps, wholesale savings must be passed on.
“If you want to save money on fuel, buy an economical car and fill it up at a supermarket.
“Although if you have a local and convenient garage that you like using, do continue to give it your support.”
RAC fuel spokesman Simon Williams said: “We badly need greater transparency in fuel retailing so that drivers pay a fair price for their petrol and diesel wherever they choose to fill up.
“When wholesale prices fall, savings should be passed on to drivers straightaway but it often takes a week or more for these to be reflected at the pumps, whereas any wholesale price increases are passed on daily.
“These ‘rocket and feather’ tactics have to disappear for good.
“While the RAC has long called for fairer pump prices, we had sympathy for smaller fuel retailers charging higher prices during the lockdown with so many drivers confined to their homes for so long.”
Industry body the Petrol Retailers Association said wholesale savings are normally passed on to customers but firms had to price fuel at pre-lockdown levels due to the drop in demand.