Fears that the credit crunch might be entering a traumatic new phase grew on Thursday as investors lost confidence in the insurers that guarantee payments on billions of dollars in bonds.

Shares in Ambac Financial and MBIA, the world’s biggest bond insurers, fell 52 per cent and 31 per cent, respectively, as Moody’s Investors’ Service raised the possibility that both might lose the triple-A credit rating on which they depend.