The US Securities and Exchange Commission has sent subpoenas to more than 50 hedge-fund advisers as part of its investigation into whether individuals spread false rumours to manipulate shares of two Wall Street firms.

The move is the latest in the SEC's campaign against short-sellers and options trading in investment banks Bear Stearns and Lehman Brothers.

It comes amid similar attempts in London by the City's regulator, the Financial Services authority, to "talk tough" about short-sellers manipulating shares in banking stocks. among the firms that have received subpoenas from the SEC, led by chairman Christopher Cox, are Citadel Investment Group in Chicago and SaC Capital advisers in Stamford, Connecticut. The subpoenas relate to trading in securities of the brokers, as well as correspondence between the hedge funds and other parties, according to people familiar with the inquiry.

Firms that have received subpoenas were told by the SEC that they were not necessarily the focus of specific allegations but part of a wider inquiry by the commission.

Bear Stearns almost collapsed in March. The SEC has been investigating whether a combination of false rumours and manipulative short-selling combined to drive down the firm's share price.