The average price of both petrol and diesel fell by more than 3p per litre over the course of April.
Data from RAC Fuel Watch shows the price of filling up a vehicle with petrol is now the cheapest it has been since May 2016.
The average price of a litre of petrol fell by 4.15p in April, from 113.1p to 108.95p, although prices are even lower at some supermarkets. The price of diesel fell by 3p, from 117.5p to 114.54p per litre.
RAC Fuel Watch says average prices dropped last month because fuel retailers were catching up with the supermarkets' massive price drops in March. Supermarkets' prices, however, remained largely static through April.
RAC fuel spokesman Simon Williams said: "While it might seem odd that fuel price falls stalled last month despite wholesale prices coming down yet further, it is explained by the simple fact that UK fuel retailers will have been selling a fraction of what they would normally have sold in the last six weeks.
"A cut in oil production, which was previously agreed by some of the world's largest oil-producing nations, is likely in time to lead to the oil price drifting back up again, although for now supply is outstripping demand so much that it's hard to see prices increasing by any great margin in May."