The falling US dollar is lowering the Organisation of the Petroleum Exporting Countries’ purchasing power by up to a third, making the powerful oil cartel more reluctant to increase production and cut prices.

Although oil is trading near last August’s record $78.65 a barrel, Opec calculations show that, when adjusted for the weaker dollar and inflation, an average of the 12 Opec members’ crude oil prices has fallen in the past year.