It may be good news for drivers filling up at the pumps, but the dramatic fall in the global price of crude could have grave consequences for the public finances of many oil-producing nations, especially against a backdrop of a broadening global economic crisis.

As they prepare to meet in Vienna next week, Opec member states are facing steep declines in tax revenues and, in some cases, rapidly swelling budget deficits.

With the oil price having slumped since July — it has more than halved, and stood at less than $70 per barrel on Thursday — an emergency meeting of the cartel of 13 producer nations is due to take place next Friday. Members such as Iran, Iraq and Nigeria will be hoping that, by implementing a cut in production, they will put a floor under recent price falls.

While wealthier producers such as the United Arab Emirates are cushioned from the falls by vast budget surpluses and foreign reserves, Iran relies on oil for 80 per cent of its revenues. Isolated from the rest of the world via a trade embargo, Opec's most hawkish member has repeatedly stated it would like to see prices stay around $100 per barrel — a level that many believe is unsustainable as the global economy sinks into recession, sapping energy demand.

“Iran has a rapidly growing, young population and oil is its only real source of revenue,” said Gareth Lewis-Davies, director of commodities research at Dresdner Kleinwort.

While nobody believes the regime of President Ahmadinejad is under threat from $70 oil, Tehran's recent efforts to raise revenues from other sources have provoked a domestic backlash. Strikes broke out last month when the Government tried to introduce a 3 per cent sales tax, forcing it to retreat. Mr Lewis-Davies said Iran needs prices to be above current levels for it to balance its budget.

Meanwhile, data from the Washington-based PFC Energy suggest that Venezuela, another Opec member, needs prices to stay about $94 to avoid sliding into a deficit.

Bayan Jabr, Iraq's Finance Minister, said recently that the decline in oil prices “no doubt will have a negative impact on Iraq's economy”, although the country has managed to build up a significant surplus this year that may shield it for several months to come.

A recent report by the National Bank of Kuwait cautioned that if spending remains high in the 2009-10 budget year, that country's budget “could slip into deficit for the first time in 11 years”, although it said it was likely to enjoy a surplus again in 2009.

However, not all member states are facing such an acute squeeze. The UAE is still announcing big new construction projects in Abu Dhabi and Dubai.

Saudi Arabia, easily the world's biggest oil producer and Opec's de facto leader, is comfortable with oil prices at a lower level. Traditionally, it has focused on a target price that is low enough to ensure high long-term global demand for oil and has shied away from forcing up prices to a level that would damage the global economy or lead to consuming countries seeking alternative fuels.

It was the Saudis who called an emergency summit to address the rising price of oil in Jeddah at the end of July.

Nevertheless, Riyadh still needs huge sums to fund its social welfare system and a slew of mammoth public-spending projects, including King Abdullah Economic City, a new urban centre to the north of Jeddah, which is being built to house two million people. It alone is expected to cost $100 billion (£60 billion).

It is the tension between Opec members over where the optimum oil price is that will dictate the outcome of next week's meeting. Saudi Arabia has so far stayed largely silent but is thought to be targeting $80 a barrel.

Mr Lewis-Davies said: “My sense is that the Saudis will be minded to cut.”.

Paul Stevens, a senior research fellow at Chatham House, said: “If they want $80 oil then they will be able to get it there. The price might wobble about a bit, but they should be able to.”

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