Investors are increasingly purchasing ultra-risky slices of complex derivatives known as "synthetic" collateralised debt obligations (CDOs), helping to double the volume of issuance of such deals last year, data today will reveal.

Sales of public and private synthetic CDOs grew to $450bn in notional terms last year compared with $224bn in 2005, according to data from Creditflux, an information and news service that has the best access to the hard-to-track private deals.