Banks may need to raise as much as $143bn (£77bn) to weather the credit crisis, Barclays Capital reports.
They say the banks will need extra money if bond insurers, who insure the products at the centre of the sub-prime crisis, lose their top credit ratings.
If their credit ratings are cut, it could make it harder for it them to pay out, leading to banks reporting bigger losses on sub-prime debt.
Fears about bond insurers helped spark off this week's stock market falls.
The world's largest banks have already admitted losing more than $100bn from mortgage bonds gone bad.