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

Fannie and Freddie shares were down by as much as 18 per cent and 23 per cent, respectively, at the peak of the sell-off after a Lehman Brothers analyst said an accounting change could, in theory, force the biggest US mortgage financiers to raise an extra $75bn in capital.