Esso has apologised to customers at the Tesco Express store at Harpenden in Hertfordshire after their cars broke down because of contaminated fuel.
An Esso spokesman said: “Unfortunately on 10 September around 30 customers purchased contaminated fuel following a delivery error which resulted in a quantity of diesel being placed into one of the unleaded 95 storage tanks at Harpenden Express service station.
“The service station was immediately shut down for investigation and no further product was sold. The contaminated fuel was removed and replaced with a fresh delivery.
“Esso wishes to apologise for any inconvenience caused to customers whose cars were affected by this incident. Site staff assisted those drivers whose vehicles were affected and we are working to reimburse affected customers.
“Every year there are over 200,000 fuel deliveries to Esso service stations across the UK and such incidents are extremely rare. We take pride in providing customers with quality fuels and take such incidents very seriously, and will investigate fully.”