Experts fear the world's biggest economy will be plunged back into recession unless President Obama and Congress reach a compromise deal that will prevent a series of expiring tax cuts and across-the-board spending reductions.
There was some encouragement from figures showing Germany's economy slowed by less than expected in the third quarter, but overall sentiment was weak as the FTSE 100 Index slipped 16.3 points to 5705.7.
In corporate news, shares in BP were broadly flat as it confirmed it was in advanced discussions with US agencies about settling criminal and other claims relating to its Gulf of Mexico well explosion in 2010.
BP, which stressed that no agreement has been reached, was 2.1p lower at 423.65p.
Centrica shares were also down, even though the company said it remained on track to meet City forecasts for profits of around £1.4 billion this year.
This will include £575 million from its residential energy supply division after it benefited from colder than normal weather at the start of the year. Shares were 1.3p lower at 317p.