Investor worries are mounting that the next big casualties from the credit squeeze might be the specialist companies that act as guarantors for bond issuers.

These companies, which write insurance to boost the credit ratings of various kinds of bonds, have seen their share prices pummelled and the cost of protecting their debt against default soar. Over the past week, sector leaders such as MBIA, Ambac, XL Capital Assurance, Radian and MGIC have all been hit hard.