Wellcome has sent out letters under section 212 of the Companies Act requiring firms to reveal the beneficial ownership of shares. Most securities firms deal through nominee names.
Advisers, led by Robert Fleming, believe some investors have gone short - sold shares they do not own - in an attempt to depress the price. Many firms want to see the market price fall so that they can buy cheaply in the offer.
Once it has the name of investors Wellcome will be able to see which firms have sold short, if any. But observers said firms would be slow to reply, making it difficult for Fleming to get the required information in time.
Wellcome Trust, which owns 73.5 per cent of Wellcome, plans to sell 330 million shares, which at yesterday's closing price of 829p, down 4p, would be worth pounds 2.8bn.