For almost two years the Opec oil cartel has watched oil prices rise with a mixture of delight and foreboding.

The group that controls about 40 per cent of the world’s total oil output, but has struggled to produce enough new oil to keep up with demand, will this year earn $1,000bn. But as high prices have begun to chip away at demand, especially in developed economies, concern has spread among Opec’s 13 members that the party is about to come to a sobering halt.