One of the largest defaults in the history of the $62,000bn credit derivatives market has been triggered by the US government’s seizure of Fannie Mae and Freddie Mac, raising questions about how dealers will unwind billions of dollars worth of contracts.

Although the $1,600bn of debt issued by the troubled mortgage groups is regarded as safe after the US government’s move to take control of the companies, their move into “conservatorship” counts as the equivalent of a bankruptcy in the credit derivatives market.