By Sara Sjolin, MarketWatch

COPENHAGEN (MarketWatch) -- The U.K. benchmark stock index fell slightly on Friday, as mining firms moved lower along with metals prices and investors remained cautious ahead of the U.S. Federal Reserve meeting next week.

The FTSE 100 index fell 0.1% to 6,583.80.

Miners posted some of the biggest losses in the index, driven lower by weaker metals prices. Shares of Anglo American PLC dropped 3.2%, Vedanta Resources PLC lost 1.9% and heavyweight Rio Tinto PLC (RIO) fell 1.4%.

Shares of Polymetal International PLC fell 3.5% after Credit Suisse cut the miner to underperform from neutral.

Credit Suisse also downgraded utility firm Pennon Group PLC to underperform from neutral. Those shares dropped 3.7%.

Bank shares also declined. Barclays PLC (BCS) lost 1.4%, Standard Chartered PLC dropped 0.5% and HSBC Holdings PLC (HBC) fell 0.1%.

More broadly, investors in London were hesitant of placing any big positions ahead of the U.S. Federal Reserve's meeting next week. Analysts speculate that the central bank could start reducing its $85-billion-a-month asset purchases after recent data have pointed to an improvement in the economy.

Outside the main index in London, shares of Imagination Technologies Group PLC jumped nearly 11% after the tech firm "significantly" expanded its strategic partnership with MediaTek.

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