Rio Tinto's defence plans against a takeover attempt from rival BHP Billiton were boosted yesterday after three Japanese conglomerates announced that they would invest in an iron ore project.

The $423m investment by Mitsui, Nippon Steel and Sumitomo Metal Industries is earmarked for the Mesa A/ Warramboo mine in the Pilbara region of Western Australia, part of a concession majority owned by the Australia-based company.