Tesco knocked 3p off a litre after oil prices fell to their lowest point in a year - forced down by fears of an international recession.

Prime Minister Gordon Brown led calls for the companies to pass on reductions to consumers.

Mr Brown said: "We are determined that, when that happens, that fall is also passed on to people at the petrol pumps and to people who have got gas and electricity bills.

"We are determined that just as people act quicklyto pass on the rise when the oil price rises, they pass on the fall when the oil price falls."

Announcing their cut, Tesco commercial director Richard Brasher said: "We know customers are felling the pinch, but we are working harder than ever in store and on the forecourt to cut the cost of living."

But the company was immediately challenged to drop its local pricing policy, under which fuel is more expensive in areas with no competition.

An Asda spokesperson said: "Having already forced prices down, we're glad to see Tesco has eventually followed our lead.

"Like always though, Tesco continues to hide behind local prcing, which means unless you're lucky enough to live near an Asda you will continue to pay too much for your fuel."

She said Asda is currently charging 105.9p for unleaded and 116.9p for diesel across all its UK forecourts.

Sainsbury's declined to be drawn into the debate and confirmed that it too was continuing with variable local pricing.

"We aim to provide our customers wuith the best possible value for their fuel, and we price competitively on a local basis," said a spokesperson.

Meanwhile, the Consumer Council highlighted the growing gap bewtween unleaded petrol and diesel at the pumps - up to 13p a litre.

"Our analysis clearly shows that diesel drivers are taking the biggest hit," said chief executive Eleanor Gill.

She said in the absence of clear evidence that such high diesel prices were necessary, it was reasonable to assume that motorists were being ripped off.

Noel Smyth, of the Petrol Retailers' Association, said his members were not reaping the rewards.

"Retailers are only getting about 4p profit on every litre," he said.

"It must be recognised that half of all filling stations have gone out of business across Northern Ireland in the past decade."

On the diesel price disparity, Paul Watters of the AA said the gap was "not a deliberate rip-off" and that there is an increasing demand for diesel from drivers.

"However there are several new refineries opening in Europe next year and we are hopeful that this will mean lower prices."

The website www.petrolprices.com shows motorists where to find the cheapes fuel in their area. On Friday it gave the following average fuel prices in Northern Ireland:

Unleaded: Belfast 110.3p, Londonderry 112.7p, Enniskillen 116.0p

Diesel: Belfast 122.0p, Londonderry 125.4p, Enniskillen 124.6p.