The changes were meant to be implemented by 6pm on Wednesday but thousands of garages appear to have not passed the cost on to motorists.
Howard Cox, founder of Fair Fuel UK blasted the move and described it as “sickening exploitation”.
He said: “I am furious that the 6p cut in fuel awarded by the Chancellor in most cases is not being passed onto drivers.
“This is a deliberate ploy by the fuel supply chain to hold onto as much profit as possible.
“This is sickening exploitation, which this government could check immediately.
“And that’s why Rishi could have implemented an independent pump pricing watchdog on Thursday at minimal cost to prevent the fuel supply chain continuing to profiteer from the world’s still highest taxed motorists.”
Tesco, Sainsbury’s, Morrisons and Asda said they would lower prices by 6p, which includes a 1p VAT reduction.
However four big brands – BP, Shell, Esso and Texaco – said they could not ensure immediate price reductions because the majority of their forecourts are run by franchisees.
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Oil prices have been particularly volatile since the Russian invasion of Ukraine began last month with investors increasingly concerned about supply.
Investors are worried that Russia, the largest oil exporter in the world, may not fulfil its contractual obligations despite assurances from the Kremlin.
Other concerns include the EU imposing an oil embargo on Russia or Moscow insisting that supplies are paid for in Rubles, a currency already unpopular with international investors which is now largely worthless due to sanctions.
However, prices appear to have stabilised and have dropped slightly since midweek due to the lack of a embargo from Brussels and increased production in Saudi Arabia.