Investors were reminded of the looming implications of further mortgage losses on Wednesday after rating agency Standard & Poor’s threatened five specialist bond insurers with losing their triple-A ratings, and downgraded a sixth to junk.

ACA Financial Guaranty was lowered from A to CCC, the lowest junk rating, after its $650m (£325m, €452m) capital cushion proved insufficient to withstand projected losses of $2.2bn under stress tests.