The crisis in US subprime mortgage-related bonds and its spillover into broader structured credit markets has had a huge impact on one important but little-known group of companies: the bond insurers.

Among the worst hit has been Radian Group, which specifically insures mortgage-backed debt. It has seen its shares tumble by more than 60 per cent this year while the cost of protecting its debt against default in derivatives markets has spiked nine times higher.