Rio Tinto and BHP Billiton have asked their Chinese steelmaker customers to accept the largest ever increase in iron ore prices by Monday or risk the interruption of supplies from Australia. Traders and industry officials said the mining companies have demanded price increases for their annual iron ore contracts in excess of the record 71.5 per cent rise of 2005 and were fighting for increases of 85-95 per cent. Rio and BHP have warned their Chinese clients some annual contracts will expire on June 30 and would cease supply under the old terms. They have told them the ore would instead be sold into the spot market, where prices are higher. Analysts said most of Rio's iron ore contracts would expire on June 30. However, some BHP contracts do not expire until September, leaving the latter more time to negotiate and allowing Rio to take the lead in the discussions. The iron ore market remains very tight and in significant deficit, with unprecedented pricing developments affecting the global steel industry and downstream sectors including magnet manufacturing.