The billionaire has pledged almost 91m shares to support loans that are unrelated to the Kazakhstan-focused copper miner. In total, Mr Kim owns 28.5pc of the group, so more than half of his stake is now backing personal borrowings. Under UK stock exchange rules, directors are required to inform senior company executives if they use their shares as collateral, because they may be forced to sell their shares to raise the funds to repay the debt. It is believed that Mr Kim has retained the shares' voting rights, but the particulars of the loans involved have not been disclosed. Mr Kim, 52, ranks at number 314 on Forbes ' list of global billionaires, with an estimated wealth of $3.5bn (2.2bn). He is the richest man in Kazakhstan and a former member of Kazakh president Nursultan Nazarbayev's political council. He has been chairman of the copper producer since 2005, when the shares listed in London. Investors have become increasingly concerned about stakes in companies being used as collateral for external loans. In 2008, David Ross was forced to resign from Carphone Warehouse, the retailer he co-founded, after using his shares as security against personal loans about which he did not inform the board. Shares in Kazakhmys have plunged by 23pc this year, as copper prices have fallen on fears of slowing growth in China, the world's largest consumer. Kazakh peer ENRC, in which Kazakhmys owns a 26pc stake, has seen its value plunge by 48pc.