A number of cars broke down after drivers filled up with fuel which is believed to have been contaminated.
Asda said it is investigating "suspected contamination" at its petrol station in Pill, Newport .
The petrol station was reported to have issues with its petrol supply on Monday night (July 1) and around five cars were left with engine problems.
An Asda spokesman said: "As soon as the problem was identified the site was closed at 7.55pm."
Craig Ash, who lives in Newport, got stuck behind some of the cars as they broke down.
The 37-year-old wrote on social media to warn other drivers. He said: "Four cars were broken down at the traffic lights directly outside the forecourt, one of them was in the middle of the cross road junctions.
"Another just up along side the passport building and another pulled up on the kerb a few moments later on Usk Way.
"I wondered what was going on then I heard one irate person on the telephone calling Asda to tell them what was happening and that they had all just filled their cars with fuel."
The Asda spokesman added: "We believe only a small number of customers have been affected and are already in contact with some of them, but if any other customers have experienced problems with their vehicles after refuelling between 7pm and 7.55pm yesterday please contact customer services on 0800 952 0101."
Gwent Police confirmed they were called to reports of broken down vehicles. A spokesman said: "Officers were called to Lower Dock Street in the Pillgwenlly area of Newport at approximately 7.50pm on July 1, 2019, following reports of a number of broken down vehicles.
"Four cars had broken down near the junction of John Street, causing a partial obstruction. Police assisted with making the vehicles and occupants safe whilst awaiting recovery."
Were you affected by the incident? Call Joshua Knapman on 02920 243847 or email joshua.knapman@walesonline.co.uk
