Chinalco, China's biggest aluminium producer, has emerged as the latest company to have a key asset tied up in the collapse of Lehman Brothers, after it said on Thursday that the failed bank was custodian of its 12 per cent stake in Rio Tinto's London-listed shares.

Earlier this month Olivant, the investment business run by Luqman Arnold, led the way when it said its entire 2.78 per cent stake in UBS (NYSE: UBS - News) was held in Lehman accounts, which also stopped it from voting the shares. Rio is currently the subject of hostile takeover interest from rival BHP Billiton (NYSE: BHP - News) , meaning voting interest on Rio's shares could be crucial. Former Lehman clients - notably hedge funds but also corporate clients - have been frantically calling for the return of their assets. However, PwC, administrator for Lehman's European operations, has warned it will likely take months to untangle the bank's holdings and commitments to a condition where they can begin returning assets.

Chinalco said there was no risk that its Rio shares would not eventually be returned as the holding was "held in a separate designated account" under a custody arrangement with Lehman Brothers International (Europe), the main operating body of the bank's European operations.

"There is no basis on which Chinalco's ownership of the shares could be validly challenged or on which its shares could form part of the general assets of LBIE," said Chinalco. It added that it was discussing with Lehman's London administrators "the arrangements for the orderly transfer of the shares out of LBIE".

Early reports that Chinalco's holding was stuck in the bankruptcy process had raised fears the Rio shares could be sold as part of any Lehman break-up, leaving Chinalco nursing a large loss. It bought the shares in February for £60 each, but Rio's share price has been badly hit by the recent market crash and closed at £20.50, down 307p.

Under UK law adminstrators have a sole duty to the creditors to maximise the value of the break-up of the estate, meaning they cannot return assets to creditors before having fully ascertained the extent of every single counterparty's net debt or credit. The Lehman collapse is already the biggest bankruptcy in history and also stands to become the most complex.

Last week the UK administrators won court approval for their proposed process for dealing with the thousands of claims from counterparties with assets held in custody by Lehmans. This week PwC asked for details of claims by each counterparty, suggesting the process is moving forward.