Global oil production will peak much earlier than expected amid a collapse in petroleum investment due to the credit crunch, one of the world's foremost experts has revealed.

Fatih Birol, chief economist to the International Energy Agency, told the Guardian that conventional crude output could plateau in 2020, a development that was "not good news" for a world still heavily dependent on petroleum.

The prediction came as oil companies from Saudi Arabia to Canada cut their capital expenditure on new projects in response to a fall in oil prices, moves that will further reduce supply in future.

Birol's comments will give more ammunition to those who warn that the British government is dangerously complacent in not trying to wean the country off oil as quickly as possible. Some observers believe that, because the global economy is underpinned by oil, the peaking of supply will cause severe economic, social and political disruption unless prepared for over many years.

John Hemming, chairman of the All Party Parliamentary Group on Peak Oil and Gas, said Birol's "conversion" was significant. "The penny has finally dropped - geological issues matter as well as political and economic. The IEA - unlike our government - appears to be leaving cloud cuckoo land finally," he added.

The IEA has never before been specific about the point at which so-called conventional oil would peak. It said last month that total crude output could peak in 2030. Birol's comments follow other signs that the IEA is rapidly changing its view. In its 2007 World Energy Outlook, the IEA predicted a rate of decline from the world's existing oil fields at 3.7%, only to admit 12 months later that the speed of the fall was more likely 6.7%.

Jeremy Leggett, chief executive of solar energy company Solarcentury, said Birol's views underplayed the scale of the problem. "The IEA is very constrained in what it can say - by the demands of its constituent governments - so you have to read between the lines. We believe that peak oil will come about in 2013 at the latest but the real concern from the IEA is the adjustment of production figures," he said.

The energy agency, which represents most western governments including the UK and US, has been backtracking rapidly on previous positions.

Three years ago the Paris-based organisation still denied there was any fundamental threat to the world's petroleum economy.