Drivers are paying more for fuel than they should because retailers' profit margins remain "stubbornly high", the competition watchdog has said.
The Competition and Markets Authority (CMA) said sellers' margins - the difference between what a retailer pays for its fuel and what it sells at - remained higher than historic levels, although fuel prices have fallen since July.
It said supermarkets' margins had risen to 8.1% in August from 7% in April.
The CMA added the "sustained" increase was concerning and there was not enough competition in the fuel market, which continued to drive prices up.