The average price of petrol climbed for the second month in a row in July amid warnings of bigger forecourt price rises.

Unleaded prices have now jumped for two months running

The average price for a litre of unleaded now stands at 145p, up a penny since the start of the month and nearly 2p since the beginning of June, RAC data reveals.

The wholesale price of petrol climbed 6p a litre from 19 July on the back of the rising cost of oil with retailers quick to pass this on to drivers as the average price has increased by nearly 2p in that time.

Diesel remained stable though at 146p (145.84p), ending a run of eight months of falling prices. As a result, a full tank of fuel for a 55-litre family car costs around £80 for either fuel – £80.21 for diesel and £79.75 for petrol.

The price of oil went up by $10 a barrel in July, rising to $85.56 by the close of the month – a price not seen since mid-April. This has caused the wholesale price of both fuels to increase – 7p for unleaded and nearly 9.5p for diesel.

While this is now squeezing retailer margins, the Competition and Markets Authority’s recent road fuel market study found evidence of supermarkets having increased such margins. And the RAC has said it’s now waiting to see how quickly prices rise and whether the supermarkets will operate with smaller margins that are closer to their longer-term averages, or whether they look to make more per litre than they have historically.

Fuel spokesman Simon Williams said: “The big question now is how quickly and how far pump prices rise. We sincerely hope we won’t see them shoot up like the proverbial rocket as this would indicate the Competition and Markets Authority’s fuel market investigation findings and recommendations have not had an immediate impact. Encouragingly, there has so far been very little upward movement from the big four supermarkets but only time will tell.”

The RAC data also shows that petrol and diesel are still considerably cheaper in Northern Ireland despite them going up by 1p and 3p respectively. Unleaded is 4p less than the UK average (£2.20 a tank) and diesel almost 5p (£2.75 a tank).

“Sadly, this shows drivers on this side of the Irish Sea are still losing out. If and when we finally see the two sets of prices coming together we hope it will be as a result of lower forecourt prices in the rest of the UK.”

The CMA has recommended major changes to revitalise competition in the road fuel sector. These include including a new fuel finder scheme that would give drivers access to live, station-by-station fuel prices on their phones or satnavs, backed by new compulsory open data requirements and a ‘fuel monitor’ oversight body.

This week has now seen Asda – which according to the CMA study had been ‘feathering’ its pump prices when wholesale prices fell – start displaying live prices online.

The RAC’s Simon Williams commented: “While it’s good to see the supermarkets start to publish their pump prices, we don’t believe displaying them on websites will suddenly make retailers start charging fairer prices, particularly as free mobile apps such as myRAC can actually compare prices among all fuel retailers – large and small – wherever they are in the country.

“Real competition is the key, and this is something we are sadly lacking at the moment, other than in Northern Ireland where fuel is more than 4p a litre cheaper than the rest of the UK.

“We don’t believe we’re likely to see real change until we have an official price monitoring body, as recommended by the Competition and Markets Authority, that has the power to fine companies that don’t properly reflect significant downward wholesale market movements on their forecourts.”