Crude oil prices dipped below the $100-a-barrel level this week for the first time since early April, in spite of Opec surprising the market by announcing a cut in oil production.

The cartel’s decision to tighten supplies – which sparked a critical response from the US government and the International Energy Agency, the energy watchdog – was seen by some traders as Opec signalling to the market that a number of its members viewed the $100 mark as an effective price floor.