The default of up to $500bn of Fannie Mae and Freddie Mac credit derivatives contracts triggered by the US government’s seizure of the mortgage groups could result in billions of dollars of losses for insurance companies and banks who offered credit insurance in recent months.

The potential losses, as well as uncertainty about exactly how the derivatives contracts will be settled and unwound, is putting strains on the unregulated $62,000bn credit derivatives market, which has been a target of regulators worried about the hidden risks it could hold for the financial system.