The Mongolian Government has expressed "great concern" at the ''diminished'' prospectivity of Rio Tinto's Oyu Tolgoi copper project, after the multinational miner delivered its final verdict on the size of its cost and schedule blow outs.

Rio announced on Wednesday evening that work to triple production of copper at Oyu Tolgoi would cost $US6.75 billion ($9 billion), a disclosure that means the project cost has blown out by $US1.45 billion.