Drivers using an Asda filling station were sold the wrong type of fuel after diesel and unleaded petrol were accidentally put in the wrong storage tanks.
The supermarket chain said it was "incredibly sorry" for the mix-up in Small Heath, Birmingham, which occurred after the forecourt closed late on Saturday October 17.
The petrol station closed at 1pm the following day after the problem was identified, and an unspecified number of customers have contacted Asda to report problems with vehicles.
It is understood diesel and unleaded fuels were put into the wrong tanks by a supplier, affecting four pumps.
An Asda spokesperson said: "This was an isolated incident, which was caused by human error.
"We are incredibly sorry for any inconvenience this mistake has caused and urge anyone who thinks they may have been affected to contact us as soon as possible."
Motorists who believe they may be affected are encouraged to call Asda's customer services line on 0800 952 0101.