Shares in Fannie Mae and Freddie Mac plunged on Monday 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 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 two biggest US mortgage financiers to raise an additional $75bn in capital.