Shares in Fannie Mae and Freddie Mac plunged yesterday as investors worried that the two giant government-sponsored mortgage financiers would have to raise fresh capital.

Fannie and Freddie shares were down by as much as 26 per cent and 29 per cent, respectively, at the peak of the sell-off after a Lehman Brothers analyst said an accounting change could, in theory, force the two biggest US mortgage financiers to raise an additional $75bn in capital.