Rio Tinto, the troubled mining group, on Thursday unveiled controversial plans to receive a $19.5bn cash injection from Chinalco and reported a 50 per cent fall in full-year profit after taking a one-off charge for writing down the value of its aluminium assets.

The planned deal, which involves the sale of minority stakes in some of Rio’s best mining assets and the issue of convertible bonds, marks the biggest ever investment by China in a foreign company.