The index was down by more than 60 points at one stage during the session but later turned higher, finishing 53.01 points up at 6,137.6.
Chris Beauchamp, senior market analyst at IG, said markets were caught in no-man’s land ahead of Thursday’s meeting.
“Ahead of what looks like the most vital Fed meeting in years, the best thing for markets to do would be simply to sit still and wait for Thursday afternoon. With that being impossible, they have opted for the second choice, namely running around in circles,” he said.
“This is not an environment for long-term decision making, indeed so much is riding on Thursday’s events the market has become even more short-sighted than normal.”
B&Q owner Kingfisher topped the blue-chip fallers’ board after posting a drop in half-year profits. Shares in the retailer slipped 9p to 351.3p.
Supermarkets also had a tough session, after the latest inflation figures showed a record run of falling food and non-alcoholic beverage prices extending to 14 months, with a 2.4 per cent year-on-year drop in August.
Tesco fell 1.4p to 177p while Sainsbury’s dropped 1p to 228.6p and Morrisons was 0.4p lower at 157.4p.
In the FTSE 250, online supermarket Ocado was ahead by 0.8p at 316.7p after it posted a better-than-expected 15.3 per cent hike in gross retail sales to £252 million in the 12 weeks to 9 August compared with a year ago.
The biggest risers in the FTSE 100 Index were Glasgow engineer Weir Group, up 60p at 1,298p, ARM Holdings up 32.5p at 980.5p, SABMiller up 81p at 3,014.5p and Coca-Cola up, 36p at 1,378p.