Petrol prices rose by 6p a litre in May – the biggest monthly increase since the RAC began tracking prices 18 years ago.
Average petrol prices hit 129.4p a litre, while average diesel prices also rose by 6p to 132.3p a litre.
The RAC said a “punitive combination” of higher crude oil prices and a weaker pound was to blame for the increases.
It pointed out that oil prices broke through the $80-a-barrel mark twice in May – a three-and-a-half year high.
As well as the higher global market price of crude, the pound’s current weakness against the US dollar also makes petrol more expensive as oil is traded in dollars.
The RAC said the average prices of both petrol and diesel had risen every single day since 22 April, adding 8p a litre in the process. The motoring body said this was the longest sustained price increase since March 2015.
RAC fuel spokesman Simon Williams described May as “a hellish month for motorists”.
“The rising oil price together with a weaker pound is a punitive combination for anyone who drives regularly,” he said.
“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.”
Mr Williams said that the oil price had since cooled a little, suggesting that the constant increases in the cost of petrol might have stopped for the time being.
He also said there was talk that the next Opec meeting in Vienna on 22 June might result in a change of policy for the oil cartel, which has been restricting supply to bolster prices.
“If a decision is taken to increase supply, it may provide some much-needed relief for motorists at the pumps in the UK,” he added.
Brent crude fell $1.10 a barrel to $75.69 on Monday afternoon.