Rio Tinto has challenged the findings of an independent report into problems at its $6.75bn copper mine in Mongolia, sticking to its explanation that difficult geology, not mismanagement, was to blame.

The Independent Consulting Group’s report, commissioned by Rio’s partners on the Oyu Tolgoi project, concluded last month that poor management was the principal reason the mine’s underground expansion was running almost two years late and $1.45bn over budget.

But in a letter to Mongolia’s justice minister Khishgee Nyambaatar, seen by the Financial Times, Rio said while it understood the government’s frustrations at the delays the ICG did not sufficiently recognise the full impact of weaker than expected rock conditions, which had meant the mine had to be redesigned.