The big four were responsible for a 3p-plus hike in fuel prices last month, according to AA Business Services.
The average price for a litre of petrol rose 3.33p in April.
However, at Tesco, Sainsbury's, Asda and Morrisons it jumped an average of 4.04p. Diesel prices rose 2.4p/litre but by 2.89p at the multiples' forecourts.
Supermarket fuel was now on average just 1.6p per litre cheaper than other forecourt operators in April. In March, it was 2.3p/litre cheaper.
The supermarkets raised prices last month because consumer confidence was returning following March's fuel contamination episode, said a spokeswoman for AA Business Services.
"They kept prices low to maintain consumer confidence and as customers have returned, prices are going back up again," she added.
The retailers blamed rising oil prices for the increase.