Mumbai: Western banks pulled down by the credit market turmoil that followed the US subprime mortgage crisis may have to raise an extra $143 billion (£77 billion) to weather the crisis, Barclays Capital said in a report.

The banks will need extra money if bond insurers, who insure the products amidst the sub prime crisis, lose their credit ratings, the report said.

A lowering of credit rating would make it difficult for the bond insurers to pay out, which in turn would cause bigger losses to banks on sub-prime debt, Barclays said.

Top banks have already reported losses exceeding $100 billion from mortgage bonds gone bad and analysts at Barclays Capital estimate total mortgage bond that banks own at $820 billion.

Bond insurers like Ambac Financial Group and MBIA, have written-off bonds worth billions of dollars in recent months and are expected to sustain more.

Rating agency Fitch cut Ambac's rating last week, while rival agencies Moody's and Standard & Poor's are reviewing Ambac's and MBIA's ratings.

Firms like Ambac are known as 'monoline' insurers and are at the centre of the sub-prime crisis.