"There is a growing consensus that the age of cheap oil is coming to an end," the UKERC study, "Global Oil Depeletion", says.

The authors warn that without early investment in strategies to reduce oil demand and deliver new energy sources, dwindling supplies of cost-effective energy from conventional oil could cramp the global economy.

The US Department of Energy has calculated that such investment would need to begin at least 20 years before oil supplies peak to avoid serious energy shortfalls.

UKERC acknowledges that there a large uncertainties in predicting the timing of "peak oil", but argues that there is sufficient information available to make an adequate assessment.

It puts the timing of "peak oil" somewhere between 2009 and 2030.

"Although this range appears wide in the light of forecasts of an imminent peak, it may be a relatively narrow window in terms of the lead time to develop substitute fuels," the authors say.

The challenge of heading off peak oil through more exploitation of existing reserves is "at best ... likely to prove extremely challenging".

The authors judge that the global oil industry needs to open up the equivalent of a new Saudia Arabia every three years just to maintain production at current levels.

More than two-thirds of current crude oil production will need to be replaced by 2030, just to prevent production from falling.

Remaining global oil reserves are now estimated to lie in a broad range between 2000-4300 billion barrels. Total global oil consumption by 2007 was 1128 billion barrels.

However, the UKERC says the timing of peak oil is insensitive to the volume of the remaining resource, and will depend as much on economic and technical factors as much as on geology.

"... annual production from a region has rarely exceeded five per cent of the remaining recoverable resources and most regions have reached their peak well before half of their recoverable resources have been produced."

Global supply is inflexible until about 2016, the UKERC report notes, because projects for the next 6-7 years have already been committed to.

After 2016, the uncertainties multiply – except that UKERC tersely observes that "... forecasts that delay the peak of conventional oil production until after 2030 rest upon several assumptions that are at best optimistic and at worst implausible".