Shares in London fell in opening trade on Thursday amid growing anxiety about the deadlock in US debt talks, dealers said.
Adding to selling pressure was a decision by Standard & Poor's to further downgrade Greece's credit rating, saying plans to restructure the country's debt would entail a default.
The benchmark FTSE 100 index was down 20.15 points or 0.34 percent at 5,836.43 at 8:28 am.
Royal Dutch Shell eased 12 pence or 0.53 percent to 2,253 pence after the global oil giant announced a 56 percent surge in underlying net profit in the second quarter.
Chief executive Peter Voser said the outlook for the rest of the year is positive.
"We have made important progress with new production in 2011, and the ramp-up of our new projects should drive our financial performance in the coming quarters," he said.
Centrica shed 2.90 pence or 0.90 percent to 317.40 pence following the utility company's announcement of a 24 percent plunge in second quarter adjusted earnings as it faced a 30 percent jump in wholesale gas prices and lower consumption.
Weapons manufacturer BAE Systems was the top riser, adding 1.74 percent to 297.60 pence. It said profit fell 22 percent in the first half but it also unveiled plans to repurchase some of its shares at a cost of up to £500 million.
Telecoms group BT added 1.73 percent to 194 pence as investors responded well to news that pretax profit jumped 20% in its first quarter to June from a year earlier.