Investors delivered a vote of no confidence in Opec last week when oil prices fell by their most in almost two decades even though the cartel decided to curb production.

Oil prices fell 26.8 per cent last week to $33.87 a barrel, down 77 per cent from the record peak in July, the steepest decline in history. Since the $100-a-barrel level was reached for the first time on January 2, crude has dropped 65 per cent, on track for a record annual decline.