Shareholders in Rio Tinto Ltd/Plc are trying to overturn a controversial $19.5 billion investment in the mining group planned by Chinese state-owned Chinalco, a report said on Sunday.

Rio plans to use the cash from the deal, which needs shareholder and regulatory approval, to pay off some of its $38 billion of debt and to expand in China and elsewhere.

A Rio Tinto spokesman declined to comment on the report, calling it speculative, but said the company remained confident the deal offered value to shareholders.

We are aware of the concerns of some shareholders and we've listened to them and explained the rationale behind the deal, the spokesman said. But we believe it's the best way forward.

Last month, Chinalco, China's top aluminum maker, agreed to pay $12.3 billion for stakes in Rio's key iron ore, copper and aluminum assets and $7.2 billion for convertible notes that could potentially double its equity stake in Rio to 18 percent.

There was speculation that Australia might block the deal, fearing Beijing would have too much influence over a major export earner.

(Reporting by Philip Waller; Editing by David Holmes)