Opec has made a scathing attack on a report from the International Energy Agency which says that the world's existing oil producers face a “huge challenge” to keep up with a projected rise in global demand.

The report from the IEA, the respected Paris-based energy advisor to the Organisation for Economic Co-operation and Development (OECD) club of wealthy nations, said that to compensate for the depletion of existing oilfields, by 2030 the world would need to find new production equivalent to 45 million barrels per day, or the output of four Saudi Arabias, to maintain present levels of supply.

It added that additional production equivalent to six Saudi Arabias would be required if a projected rise in oil demand from 85 million barrels a day to 106 million was taken into account.

The IEA, which based its findings on a landmark study of decline rates at 800 of the world's largest oilfields, said that there was, in theory, enough oil left in the ground to meet demand. However, it would require investment of about $450 billion (£300 billion) a year, with the bulk of this spent in the 13 member states of Opec, where most of the world's remaining supplies lie.

Abdullah al-Badri, Opec's secretary-general, gave a withering verdict on the study. “I don't trust this report,” he said. “I don't think the IEA is equipped to review these oilfields...I don't see how it will be useful.”

He said that Opec, the cartel of 13 oil-exporting countries that produces 40 per cent of the world's oil, had not been involved in drafting the study, which he dismissed as alarmist. “We have the reserves, we have enough oil for the foreseeable future,” Mr al-Badri insisted.

Speaking at the London launch of the IEA's 2008 World Energy Outlook report yesterday, Nobuo Tanaka, the agency's executive director, a leading energy expert, adopted a markedly different tone, sounding a grave warning about the energy challenges facing the world. “Current trends in energy supply and consumption are patently unsustainable - environmentally, economically and socially - they can and must be altered,” Mr Tanaka said. He added that an “energy revolution” was needed urgently to meet surging global demand, which he predicted would soar by 45 per cent by 2030 - driven mainly by China and India - while also adopting low-carbon sources of power to prevent catastrophic climate change.

The dispute between the IEA and Opec goes to the heart of the debate over “peak oil” and how much of the world's energy needs its existing oilfields can supply in the years ahead. This year's World Energy Outlook report slashed its assessment of how much oil the world would be able to produce by 2030 by ten million barrels to 106 million per day and placed more emphasis than ever before on the need to develop alternatives.

Opec has traditionally adopted a much rosier view of the prospects for future global oil production growth. For years, it has also been accused of overstating its reserves for political reasons and to discourage the development of alternatives.

The IEA's report also gave warning that the present economic slowdown could have damaging consequences for the world's energy supplies by undermining crucial investment. “We cannot let the financial and economic crisis delay the policy action that is urgently needed to ensure secure energy supplies and to curtail rising emissions of greenhouse gases,” Mr Tanaka said. “We must usher in a global energy revolution by improving energy efficiency and increasing the deployment of low-carbon energy.”

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