The average prices of petrol and diesel have increased for the third month in a row, making filling up an average family car around £5 more expensive than at the end of February.

RAC Fuel Watch data for May reveals that the average unleaded pump price rose 2p a litre from 108.67p a litre to 110.59p, adding £1 to the cost of filling up a 55-litre family car in the course of the month. It also means that since the end of February the cost of a tank of petrol has risen by £4.77.

The average diesel forecourt price saw an even greater increase, going up by 2.2p a litre from 108.49p to 110.70p. This made the cost of a full tank of diesel £1.22 more expensive (£59.67 to £60.89), and a whopping £5.14 dearer than at the end of February.

RAC fuel spokesman Simon Williams said: 'Motorists will be disappointed to see pump prices going up again having enjoyed filling up their vehicles with petrol and diesel for under £1 a litre at the start of the year. The sub-£1 litre is now sadly starting to seem like a distant memory.

'Since the end of February the price of both unleaded and diesel have increased by around 9p a litre which has driven the cost of filling up by around £5. Now summer's here what most families want is a period of stability so that making the most of the better weather doesn't lose some of its shine through higher costs at the pump.

'Fortunately, some of the refinery issues which had led to lower-tha-expected output have been resolved and we now have increased crude production in the Middle East.

'And, with the Organisation of the Petroleum Exporting Countries [Opec] deciding not to curtail production at its six-monthly meeting in Vienna, there is good reason to expect the oil price will not go far above the current level for the time being, but predicting what will happen with oil is far from straightforward.'