The citizens of Seine-Saint-Denis, a poor suburb with some of Paris’s grimmest housing estates, seem unlikely speculators in credit derivatives. But that is what they have become thanks to the Communist party of France, with help from some of the nation’s top banks.

The district, north-east of the capital, was for decades a Communist stronghold. Nonetheless, several years ago, the council decided to restructure its large debts by taking out complex financial products – the riskiness of which has now been exposed by the global crisis.