By Sara Sjolin, MarketWatch

LONDON (MarketWatch) -- U.K.'s benchmark stock index was on track for a third straight day of losses on Wednesday, with resource firms tracking most commodity prices lower.

The FTSE 100 index dropped 0.5% to 6,420.07, on track for its lowest close since early July.

Shares of BHP Billiton PLC (BHP) posted one of the biggest losses in the index, off 2%, as most metals prices were on the decline. Rio Tinto PLC (RIO) lost 1.8% and Anglo American PLC gave up 1.1%.

Oil firms were also mostly lower, as oil prices fell below $105 a barrel. BP PLC (BP) dropped 1%, BG Group PLC lost 0.7% and Royal Dutch Shell PLC (RDSB) fell 0.3%.

Shares of HSBC Holdings PLC (HBC) dropped 1.5% as the stock traded ex-dividend, meaning new investors will miss out on the latest dividends.

British American Tobacco PLC also traded ex-dividend, pushing the shares 0.7% lower.

Also on the decline in London, shares of chip maker ARM Holdings PLC (ARMHY) shaved off 0.9%.

On a more upbeat note, shares of Standard Life PLC rose 1.8% after Credit Suisse lifted the firm to neutral from underperform.

"Costs are being well contained and offer support for net margins, while the macro backdrop continues to appear supportive," the analysts said.

Also higher in London, shares of Lloyds Banking Group PLC (LYG) rose 0.6% after the bank said it will sell its German life insurance business Heidelberger Lebensversicherung AG for 300 million euros ($402 million) in cash. Additionally, Lloyds said it will sell off a loan portfolio to Goldman Sachs Group Inc. (GS) for 254 million pounds ($398 million) in cash.

Outside the main index, shares of N Brown Group PLC jumped 4.4% after Credit Suisse lifted the home-shopping firm to outperform from neutral.

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