Ratings agency Moody's has said it may cut the US AAA debt rating, pointing to the "rising possibility" the US will default on its debt obligations.
The FTSE 100 fell below the 6,000 level in mid-morning trading as European markets absorbed Moody's downgrade of Portuguese debt to 'junk' status.
Moody's has downgraded Portuguese debt to 'junk' status on fears it will need a second bail out just months after it received €78bn.
Three major French banks could have their credit ratings cut by Moody's because of their exposure to Greek debt.
Fears the UK government may undershoot its target to cut the budget deficit could threaten the country's credit rating, Moody's has warned.
Moody's Investors Service last night said if there is no progress in the US on increasing the country's statutory debt limit, it expects to place that government's rating under review for possible downgrade in the next few weeks.
Moody's said it has put Citigroup, Bank of America, and Wells Fargo on review for possible credit ratings downgrades as the US government cuts support for future bailouts.
Iceland is poised to re-enter the bond market for the first time since the collapse of its major banks.
Ratings agency Moody's is threatening to cut the ratings of 14 British lenders including RBS and Lloyds on concerns regulators including the Bank of England will harden their stance on future bailouts.
Moody's has downgraded its rating of Irish banks to junk status, following its decision last week to cut Ireland's sovereign debt rating by two notches.