The blue-chip index fell 26.35 points, or 0.5 per cent, to 5,410.35 -- below the 5,450 level it has managed to close above just once since early August -- as investors pulled out of the mining and banking sectors.
The FTSE bounced off a session low at 5,348.64, tracking movements on Wall Street as market volatility continued in thin trade.
Rolls Royce and IMI were among the top risers up 1.6 per cent and 2.2 per cent, respectively.
Elsewhere, Whitbread rose 0.5 per cent after Britain's biggest hotel and coffee shop operator reported a higher-than-expected first-half pretax profit and hiked its dividend over 50 per cent.
Bargain hunters picked up G4S, which bounced 9.8 per cent, having slumped more than 20 per cent on Monday when announcing a deeply discounted rights issue to pay for its £1.5bn acquisition of Danish firm ISS.