Many motorists are missing out on the 1p a litre fuel duty reduction announced by the Chancellor in the Budget, according to AA figures.
Average petrol prices have only dipped 0.6p a litre following George Osborne's decision, while average diesel prices have only gone down 0.57p.
The 1p cut should have taken effect from 6pm on Wednesday.
On Friday the AA said that, as of Thursday, the average petrol price was 132.93p a litre compared with 133.53p a litre on Wednesday before the Budget. It added that diesel had fallen from 140.26p a litre on average on Wednesday to 139.69p on Thursday.
The AA said Shell had passed on the cut at its service stations, as had supermarket Morrisons
AA president Edmund King said: "The latest fuel prices clearly show that not all garages have passed on the Budget's 1p cut in fuel duty. Drivers have long memories and will remember those garages that have not passed on the cut.
"Prices falling three-fifths of the Chancellor's duty cut rams home what the AA has been saying since 2005 - the market needs transparency to protect the interests of drivers, retailers and suppliers."
Prime Minister David Cameron urged garages to pass on the tax reduction and said there were ways to ensure "this market operates properly".
Speaking in Brussels, he said: "Compared with what was going to happen it's a 6p reduction. And of course I want to see that passed on by garages. We have done what we can as a government to cut tax, it is now right that the market should respond.
"If the market doesn't respond, obviously there are ways for the Office of Fair Trading and others to make sure this market operates properly. We will be watching like a hawk to make sure the action we take actually helps consumers and helps motorists at the pump."