Supermarkets and other fuel retailers have until August to voluntarily share live prices in a government scheme designed to prevent overcharging.
It follows a report that found drivers paid an extra 6p per litre for fuel at supermarkets last year due to weak competition.
Drivers will be able to compare live prices online to find cheaper fuel.
But campaign group FairFuelUK said the “jury was still out” on the scheme’s effectiveness.
Energy Secretary Grant Shapps will meet bosses from Asda, Tesco, Morrisons, Sainsbury’s and other major fuel retailers on Monday, who will be “held to account for any failure to pass on savings to consumers”.
Announcing the scheme earlier in July, Mr Shapps said the government wanted to “shine a light on rip-off retailers”.
The secretary of state will give retailers until next month to agree to the voluntary Competition and Markets Authority (CMA) scheme to share live fuel prices, in order to improve transparency and increase competition.
Retailers are being asked to share up-to-date fuel prices voluntarily by August as an interim measure before a new law is introduced to force companies to publish live data.
The government wants the pricing data to be available to third parties such as satnavs or map apps.
It plans to consult on the scheme this autumn ahead of new laws being introduced. The CMA has also recommended that there is a “fuel monitor” body to oversee the scheme.
Petrol and diesel prices jumped to record highs following Russia’s invasion of Ukraine last year, but have since dropped significantly.
However, the CMA, the UK’s competition watchdog, investigated the UK fuel market earlier this year following concerns falling wholesale prices were not being passed on to consumers.
It concluded competition was “not working as well as it should be”.
The CMA found average annual supermarket margins on fuel had increased by 6p per litre between 2019 and 2022 – equivalent to £900m in extra costs for drivers.
RAC fuel spokesperson Simon Williams said: “What’s badly needed is an official wholesale fuel price monitoring function which has the power to fine or take action against major retailers who don’t lower their forecourt prices when wholesale costs drop significantly.
“While the CMA recommended an element of monitoring wholesale prices in its report in UK fuel retailing, the RAC fears without the threat of consequence in the form of fines, the biggest retailers are unlikely to lower their pump prices quickly enough when the wholesale market trends down.”
