The Opec cartel gave warning yesterday that the collapse in crude prices had already put some big oil projects in jeopardy - hinting that future supplies were in danger. It raised the alarm as it slashed production in an effort to put a floor on plummeting prices.

The oil producers' cartel reduced its official quota level by 1.5m barrels a day from November 1. But prices shrugged off the news and continued to fall, reflecting the severity of the economic crisis and the speed with which it was spreading to important oil-demand growth areas such as China.