Tesco has apologised to customers after petrol was put into diesel tanks at the firm's petrol station in Hayes.
A spokeswoman for Tesco said: "We are sorry to the small number of customers affected by this mistake.
"As soon as the mistake was spotted both tanks were taken out of service and we are working with the affected customers to resolve any problems.”
Supermarket chain Asda is to cut its fuel prices by up to 2p a litre.
From tomorrow, drivers filling up at any of Asda's 195 forecourts nationwide will pay no more than 136.7p a litre for petrol and 141.7p a litre for diesel.
Sainsbury's has also said it would, from tomorrow, be cutting its petrol and diesel prices "in many of our stores by up to 2p a litre".
Firefighters were called to deal with a fuel spillage after a car was overfilled with petrol in Deerswood Road, Crawley, West Sussex. Around a litre of petrol spilt on to the road, which was sanded by crews from West Sussex Fire and Rescue Service just after 3pm yesterday, the brigade said today.
There are long queues at petrol stations in Harpenden, Herts.
Many pumps are now dry.
Police are having to control queues on the main roads as other road users try to overtake them.