Days after the Government accused the big four firms of fleecing motorists, a study said pump price falls slowed again in the past few weeks.
Petrol is up by a penny to 143.0p on average — the first rise in eight months — despite wholesale costs dropping, the RAC’s Fuel Watch found.
It is still 5p per litre more than it should be, with diesel 6p dearer.
Ministers are now supporting plans for a regulator to make sure firms are not ripping off drivers.
There are also plans for an online app or comparison tool so Brits can log on and find the cheapest fuel in their area – saving them £4.50 a tank.
The MyRAC app already exists which can save Brits up to 6p per litre by shopping around.
RAC fuel spokesman Simon Williams said: “June marked the end of the price of petrol falling at the pumps, purely because retailers are taking more margin per litre than they used to.
“Looking at the wholesale price of both petrol and diesel which is almost identical, average forecourt prices should be 5p lower for petrol and 6p for diesel.”
June also saw a huge difference between Northern Ireland and the rest of the UK – with petrol there around 4p a litre cheaper, and diesel 7p per litre cheaper.
Mr Williams added: “These differences are quite shocking considering all retailers pay the same wholesale prices.”