Petrol prices are on the rise again after more than three months of falling cost at the pumps, the AA has warned.

The motoring organisation also attacked the fuel industry for failing to pass on fully a 2p fall in diesel wholesale costs in November and December that, with VAT, were worth 2.5p at the pump.

The attack came as the Petrol Retailers Association told the Daily Telegraph that wholesale prices had risen by 5p a litre since Christmas and wholesale cost changes could see them go up by another 4p in coming days.

PRA chairman Brian Madderson said: "Independent retailers have been soaking up this increase at the expense of already tight margins because they know how hard the motorist is squeezed. But the floodgates will have to open soon."

The AA claims come ahead of the publication at the end of the month of the findings of an Office for Fair Trading (OFT) review into whether reductions in the price of crude oil are being passed on to motorists.

Edmund King, the AA's president, said: "Another new year, another new round of pump price rises after the industry failed to pass on fully wholesale price savings. The Office of Fair Trading decides soon whether to launch an investigation into fuel prices, hopefully tackling the fuel industry's treatment of drivers, consumers and businesses.

"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."

In its latest price report the AA said a rise of 0.75p since the new year left average prices at 132.71p a litre. Diesel is marginally cheaper than a month ago, falling by a third of a penny in the third week of December and remaining around 140p per litre throughout Christmas and early January.

But the average of 140.32p a litre, compared to 140.38p in mid December means a litre of diesel has been on average 8p a litre more expensive than petrol, compared with a 5p difference in the summer, something the AA blames on the drop in wholesale costs not being passed on.

Since the start of the new year, the pound has weakened against the dollar in international trading, putting pressure on prices to move upwards. Average petrol prices are unchanged in Wales but up 0.7p a litre in the West Midlands and East Anglia. Average diesel prices show little change month on month, apart from Scotland where average prices are down 0.7p a litre. Yorkshire and Humberside remains the cheapest area in the UK for petrol and diesel at 132.1p and 139.7p a litre respectively, with Northern Ireland the dearest at 133.6p and 141.0p per litre.