The FTSE 100 had been up more than 40 points at one point, but finished 5 points lower at 6,100.
Mining stocks saw the biggest losses, with Glencore down 5% and Anglo American dropping 4.5%.
The top rises included High Street stalwart Marks & Spencer and Primark owner AB Foods, both up about 3%.
Investors were cautious before a speech from US Federal Reserve chair Janet Yellen due later in the day.
Markets will be examining Ms Yellen's comments for clues as to when the Fed might next raise interest rates.
(Close): After a strong start to the day, the FTSE 100 fell back to close little-changed, with mining stocks weighing on the market.