Shares in Swiss Re tumbled on Wednesday after the reinsurer said it had $9.6bn of exposure to corporate debt issued by Freddie Mac and Fannie Mae, the beleaguered US mortgage finance companies.

Swiss Re fell SFr1.45, or 2.3 per cent, but was down as much as 9 per cent during the day as investors worried the reinsurer could face more writedowns related to its exposure to Fannie and Freddie.