The RAC believes the tumbling oil price will lead to a 3p drop for petrol and 5p for diesel. This would take average prices to around 103p for petrol and 104p for diesel, but experts predict prices will be lower at many forecourts. RAC spokesman Simon Williams said: “We expect Britain’s supermarkets and cheapest fuel retailers to be selling petrol at £1 a litre or less in time for Christmas. “These retailers consistently tend to be 3p-5p a litre cheaper than the UK average price. We are still some way off the average price of unleaded reaching the £1-a-litre mark, but this will be a big step in the right direction.” Tesco and Asda announced on Tuesday that they have slashed petrol and diesel prices by 2p a litre, while Sainsbury’s said it would cut its prices by “up to 2p”. Recent savings have been attributed to oil prices slumping to a near seven-year low, with a barrel of Brent crude reaching 41 US dollars earlier this week. Twelve months ago a barrel cost 65 US dollars and in June last year it was 115 dollars. Mr Williams added: “If retailers don’t pass on the savings quickly, they will be giving themselves an unpopular Christmas boost to profits by pocketing the extra margin. They should really be passing this on to their customers instead. “The long-term outlook is for the oil price to stay low.” Last month saw a brief return to petrol being sold for under £1 a litre with Asda cutting its price to 99.7p in a three-day promotion. The average price of diesel is already at a six-year low, dropping to 109.18p on Friday.