Motorists suffered the biggest monthly rise in the average price of petrol in at least 18 years with 6p a litre being added at the pumps in May, new data from the RAC reveals.

The average cost of unleaded fuel shot up from 123.43p to 129.41p per litre, taking the cost of filling up a 55-litre family car to £71.18 – an increase of £3.29 in just one month.

The average price of a litre of diesel rose by 6.12p, from 126.27p to 132.39p. This is the second biggest rise since the start of 2000 – when the RAC started recording the data – but still some way behind the 8.43p increase seen in May 2008 (120.83p to 129.26p).

Neither unleaded nor diesel have been priced this high since 2014. Unleaded was last at current levels at the start of October 2014 whilst diesel was last at current levels in mid-August 2014.

In May the big four supermarkets raised petrol by 5.49p a litre and diesel by 5.88p on average. On the motorway, service stations added 6.37p to unleaded, taking it to 144.75p a litre, and 6.69p to diesel making it 147.80p a litre – 15p a litre above average UK prices for both fuels.

Data from the RAC also shows the average prices of both petrol and diesel have gone up every single day since 22 April – the longest sustained price increase since March 2015.

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How do the price changes vary by region?

The north west of England experienced the biggest rise in petrol prices with a litre going up 5.98p in May to 128.72p. Northern Ireland saw the smallest increase at 5.07p a litre. The cheapest litre of petrol could be bought in the north east of England where it cost 128.24p at the end of May. The south east was the most expensive place to buy petrol with a litre averaging 129.83p.

With regards to diesel, drivers in Wales endured the greatest increase on the forecourt with a litre jumping by 6.07p to 132.03p. The north east and south east had the smallest rise at 5.66p, but a litre of diesel in the south east was the most expensive in the UK at 132.73p.

What does the RAC say?

RAC fuel spokesman Simon Williams said: "May was a hellish month for motorists. Sadly, they have been besieged by pump price rises for three months with nearly 9p a litre being added to petrol since the beginning of March.

"The rising oil price together with a weaker pound is a punitive combination for anyone that drives regularly. For many people there is little alternative to the car for the majority of journeys they have to make so it is therefore very difficult to avoid feeling the pinch of rising pump prices."