The UK's blue chip index traded 0.84 per cent down at 5,979 points.
"Worried British investors have half an eye on Brussels. Prime Minister David Cameron’s level of accomplishment in negotiating a better deal for the UK in Europe with 28 of his European counterparts could have a strong bearing on the direction of UK asset prices," said Jasper Lawler, market analyst at CMC Markets.
"Consensus in markets is that with unemployment at near five per cent, wages growing and consumer confidence relatively high, the British people won’t want to rock the boat by voting to leave the EU," he added.
"But if Mr Cameron’s deal is perceived to be weak, it will pose a distinct risk to this view and could and to uncertainty in the lead-up to the referendum."
Anglo American was the largest faller, dropping 6.22 per cent to 438.95p per share. BHP Billiton's share price slid 0.77 per cent to 737.3p.
And "the recovery in oil could not prevent Royal Dutch Shell from losing", said Ipek Ozkardeskaya, market analyst at London Capital Group. Its share price fell 2.41 per cent to 1,596.5p per share. BP was down less sharply by 0.97 per cent to 346.45p per share.
Despite the index falling as a whole, BAE Systems was one of the largest risers, up 1.94 per cent to 509p per share. This morning the company reported revenues had grown, with prospects for future orders strong.
And British Gas parent Centrica, which also reported today, rose 6.39 per cent to 206.5p per share in early morning trading. The company cut its dividend as it reported revenue and profit to have fallen.
But Vodafone's share price fell 0.58 per cent to 216.05p after the telecoms company announced it is aiming to raise £2.9bn by issuing mandatory convertible bonds.
Pharmaceuticals also fell. AstraZeneca fell 0.54 per cent to 4,126p per share, while GlaxoSmithKline's share price dropped 0.22 per cent to 1,378p. Hikma Pharmaceuticals was down more sharply by 1.99 per cent to 1,874p.