Institutional Shareholder Services has become the second big governance adviser to recommend Rio Tinto investors back a review into the miner’s dual-company structure despite a warning from directors that its ASX-listed shares could lose 11 per cent of their value upon unification.

The proxy advisory firm told clients on Tuesday night to support the resolution filed by British hedge fund Palliser Capital, which seeks to compel Rio to conduct a review of the structure that has prevailed since the 1995 merger of Australia’s CRA and London’s RTZ Corporation.