By Sara Sjolin, MarketWatch

LONDON (MarketWatch) -- Earnings reports weighed on some of the FTSE 100's biggest companies on Thursday, leading the benchmark index to a third straight day of losses.

The FTSE 100 lost 0.9% to 6,739.75, building on a 0.5% loss from Wednesday.

Shares of Royal Bank of Scotland Group PLC (RBS) topped the list of decliners, sliding 7.4%, after the 81% state-owned lender posted a loss of GBP9 billion ($15 billion) for 2013 and announced a plan to cut GBP5 billion in costs over the next few years.

RSA Insurance Group PLC lost 3.1% after the insurer launched a GBP775 million rights issue to strengthen its balance sheet. The company also reported a sharp decline in operating profit, which is a key measure in the insurance industry.

Another insurer, Standard Life PLC , lost 2.6% after the company reported a 30% drop in full-year profit.

WPP PLC slumped 5.3% after the advertising giant said it missed its margin target for 2013. The slide in many emerging-market currencies against the pound weighed on profit in the second half and particularly in the last quarter of the year, the company said.

On a more upbeat note, shares of Reed Elsevier PLC advanced 1% after the publishing firm reported a rise in full-year earnings and raised its dividend.

Outside the main index, Man Group PLC rallied 11% after the investment manager said it swung to a full-year profit in 2013 and laid out plans to repurchase $115 million of its ordinary shares to return surplus capital to shareholders.

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