A former worker on Rio Tinto's Oyu Tolgoi copper project has claimed in a United Kingdom court that he warned senior Rio officials about cost and schedule blowouts months before the company disclosed the issues to investors.

The worker was a general manager on the Oyu Tolgoi expansion in Mongolia, which Rio announced in July would cost between $US1.2 billion and $US1.9 billion more than previously expected, and be delivered between 16 and 30 months later than scheduled.