Copper production from Rio Tinto’s most important growth project will not increase as rapidly nor efficiently as expected, after tensions with the Mongolian government forced Rio to make a major engineering change at the Oyu Tolgoi mine.

Rio has been forced to halt eight months of tunnelling work to move the giant underground mine in a northerly direction because the Mongolian government has delayed approval for the mine to enter tenements that are partly owned by Canadian company Entree Resources.