Fuel retailers have suggested that rising prices at the pumps are due to "speculation" by traders in the wholesale market.
Petrol prices have been going up after more than three months of falling costs at forecourts and the AA has accused retailers of failing to fully pass on a 2p fall in diesel wholesale costs in November and December that, with VAT, were worth 2.5p at the pump
But the Petrol Retailers Association (PRA) said the hike in petrol costs was due to an unexplained leap in wholesale prices of 5p a litre since Christmas and warned that wholesale cost changes could see them go up by another 4p in coming days.
It comes ahead of a review by the Office of Fair Trading (OFT) into whether reductions in the price of crude oil are being passed on to motorists, which will be released next week.
Brian Madderson, chairman of the PRA, said: "We cannot explain to our customers why the wholesale price is going up so much - it is not due to Government tax, it is not due to Brent crude going up and it is not due to the weak currency exchange.
"So are traders, bankers or speculators taking British motorists for a ride? Motorists will understand that, in this winter weather, demand for petrol in this country has been at a low ebb. If there is no massive demand, why have wholesale costs been ramped up by nearly 5p a litre? We do not understand it - yet once again we are going to be accused of profiteering at the pumps when that is simply not true."
Mr Madderson called on the OFT to launch an investigation into the wholesale market rather than target petrol stations. He also accepted that retailers had failed to pass on the 2p fall in diesel wholesale costs ahead of Christmas, but said that was due to the "horrendous time" they had during the summer.
According to the latest figures released by Experian Catalyst, petrol is selling on average for 132.83 pence a litre and diesel at 140.4 pence a litre.
Edmund King, president of the AA, said: "The insight we are now getting on wholesale price movements rams home the need for this information to be out in the public domain immediately. Wholesale petrol prices turned upward in the first week of January, average pump prices six days later. If falls in wholesale were reflected as quickly, no one would mind - but they're not."
RAC technical director David Bizley said: "It's hugely disappointing for motorists that another painful price rise is looming, particularly as it comes so quickly after the good news that the Government has scrapped the planned January 3p fuel duty increase. We hope the OFT's findings will finally bring some much-need transparency to fuel pricing. While people understand petrol retailers' need to raise prices when industry wholesale costs go up, it is extremely frustrating to watch prices go up far faster than they ever come down."