Kazakhmys has come under fire from an anti-corruption watchdog over fears that it is controlled by Kazakhstan's totalitarian ruler.
According to the mining firm, chairman Vladimir Kim owns nearly 28 per cent of the company, much of it via a trust in Liechtenstein, while chief executive Oleg Novachuk owns 5.5 per cent through a trust registered in the British Virgin Islands.
Kim alone earned nearly £20m in dividends from his stake, the lion's share of which went through Liechtenstein, a well known tax haven.
Both jurisdictions do not require companies to disclose who their ultimate beneficiaries are.
Anti-corruption group Global Witness said the company had refused to provide evidence that the beneficiary was not autocratic president of Kazakhstan, Nursultan Nazarbayev.
'London is seen as a soft touch as a listing location,' said spokesman Tom Mayne, adding that this left pension savers exposed to unknown risks.
Kazakhmys (up 33.5p to 1250.5p) insisted that it had undergone comprehensive due diligence before listing in 2005, adding that it was 'beyond reasonable doubt' that the chairman and chief executive were the beneficiaries of the shares.
The Kazakh government already owns 26 per cent of the company directly.
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