Sigma Finance, a $40bn-plus complex debt investment vehicle run by the London-based specialist asset manager Gordian Knot, has been put on review for downgrade by Moody’s investors Service, the ratings agency.

The agency said that $27bn (£13.6bn) worth of senior short- and medium-term debt issued by Sigma, Gordian’s only significant business, were set to lose their top-notch credit ratings because of falling asset values and its inability to issue new debt.