Petrol prices in May saw the highest monthly increase since the RAC began tracking prices 18 years ago, with average prices up 6ppl.

Over the month unleaded rose from 123.43ppl to 129.41ppl.

The increase in the average price of a litre of diesel was slightly greater at 6.12p – from 126.27ppl to 132.39ppl – but this was the second greatest monthly rise since the start of 2000, as diesel shot up 8.43ppl in May 2008, from 120.83ppl to 129.26ppl.

The RAC Fuel Watch, based on Experian Catalist data, also shows the average prices of both petrol and diesel have gone up every single day since 22 April, adding 8ppl in the process – the longest sustained price increase since March 2015.

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

The increases have been driven by a jump in the price of oil, coupled with a weakening of the pound against the dollar. Oil broke through the $80 a barrel mark twice in May – something which has not been seen for three and a half years (since 12 November 2014).

Overall, May saw a 3% increase in the price of a barrel of oil (from $74.31 to $76.39) and a 2% drop in the value of the pound against the dollar ($1.35 to $1.33), making the wholesale price for retailers more expensive as fuel, like oil, is traded in dollars.

RAC fuel spokesman Simon Williams said: “In the last week of May the oil price cooled a little to $76 a barrel, which is slightly better news for motorists as the RAC’s two-week forecast is currently showing that average prices may even reduce by a penny or so. While this isn’t much, and could easily change in response to oil trading this week, it is at least a sign that the constant rise in forecourt prices may have stopped for the time being.

“Looking at the bigger picture, there is talk that OPEC – the Organisation of the Petroleum Exporting Countries – may agree at its meeting on 22 June in Vienna to change its tack of restricting oil supply. The group, together with Russia, has been limiting production with a view to removing the long-term oil glut. This strategy has been successful and, as intended, caused the barrel price to rise. If a decision is taken to increase supply it may provide some much-needed relief for motorists at the pumps in the UK.”

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