Swiss banking regulators raided the offices of Deutsche Bank in Zurich last night in an investigation into options trading connected with the purchase of a significant stake in Sulzer, the Swiss engineering group, by companies connected to the Russian oligarch Viktor Vekselberg.

The Swiss Federal Banking Commission also searched offices of Zürcher Kantonalbank and Neue Zürcher Bank, seeking evidence of possible violations of share-disclosure rules.

Mr Vekselberg, who is a large shareholder in the TNK-BP joint venture with the British oil giant, acquired a 32 per cent interest in Sulzer jointly with Victory, an Austrian conglomerate. The share stake is believed to have been acquired using cash-settled options, a form of derivative that has enabled investors to build up controlling stakes in Swiss companies without making standard share-purchase disclosures.

Renova, Mr Vekselberg’s company, revealed last month that Everest, a company jointly controlled by Victory and Renova, had acquired 18 per cent of Sulzer plus options over a further 14 per cent. The disclosure followed weeks of speculation and rumour that Sulzer was being stalked by a bidder.

In 2005 Victory took over Oerlikon, another Swiss engineering firm, gradually acquiring a 49 per cent interest without disclosure by using cash-settled options. The subtle, backdoor takeovers have created a furore in the Swiss financial community, amid calls for reform of the rules pertaining to share disclosure.

Ulf Berg, the chairman of Sulzer, protested against stakebuilding by stealth, saying that it was “deplorable” that an investor could pass through four disclosure levels without having to give notification of its interest.

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