Public pressure for petrol price cuts has prompted Prime Minister Gordon Brown to urge oil companies to follow the example of two supermarkets in reducing the cost of fuel.
Asda and Morrisons announced they were reducing the price of unleaded to 99.9p a litre on Wednesday - the first time since December that the level has fallen below £1.
Mr Brown, who was in Brussels for an EU summit dominated by the global credit crunch, said the price cut was not being passed on to all consumers.
"We will be monitoring what is happening and I expect other companies to follow the path being taken by two supermarkets," he said.
"People know that when oil prices go up, that is reflected very quickly in the pump price. And they want to know that when it goes down, that is also reflected in the price."
World oil prices have been tumbling in recent days amid concerns over weakening demand, which is now at around half the high point achieved over the summer.
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