London shares fell in opening trade on Thursday after Moody's warned it could downgrade the United States' top-class debt rating.
The FTSE 100 index of leading companies was down 0.84 percent at 5,856.42 points in early deals.
Credit ratings agency Moody's placed Washington's sterling triple-A debt rating on a downgrade watch late Wednesday, raising fears of a default by Washington.
Moody's blamed US lawmakers' failure to hammer out a deal that would allow President Barack Obama to raise the country's debt ceiling, in turn paving the way for it to meet its repayment obligations.
With trillions of dollars of US debt held by countries and corporations around the world, a US ratings downgrade would likely send global markets into a downward spiral.