The least risky mortgages in the US are showing the sharpest jump in "serious delinquencies", according to a report released yesterday that raised fresh concerns about the health of the US economy.

The report showed that credit quality in all loan categories continued to decline in the fourth quarter of 2008, with the highest risk subprime mortgages showing, as expected, the highest level of serious delinquencies - defined as loans in arrears for 60 or more days.