By Sara Sjolin, MarketWatch

LONDON (MarketWatch) -- The U.K.'s benchmark stock index closed marginally higher on Thursday, holding on to a four-week high as supermarket firms rose after a well-received earnings report from Wm. Morrison Supermarkets PLC.

The FTSE 100 index edged up less than 0.1% to 6,588.98, closing at the highest level since Aug. 13.

Shares of Wm. Morrison Supermarkets rose 1.8% after the supermarket chain said it expects sales to improve in the second half and full-year performance to be in line with its previous expectations. Additionally, the food retailer lifted dividends 10%.

Other supermarket chains were also on the rise, with shares of J Sainsbury PLC up 1.2% and Tesco PLC (TSCDY) rising 0.1%.

Shares of AMEC PLC put on 1.9% after the oil-services firm said it has no intention of making an offer for Kentz Corp. Ltd. . Kentz shares dropped 9.2%.

Some banks were also higher, with shares of Royal Bank of Scotland Group PLC (RBS) up 1% and HSBC Holdings PLC (HBC) 0.3% higher.

Shares of Aggreko PLC fell 2.5% to 16.10 British pounds ($25.48) after Deutsche Bank cut the price target on the temporary-power provider by 11% to GBP18.70. The analysts said pressure from emerging markets is likely to continue to weigh on profitability in the near term.

Shares of Melrose Industries PLC fell 1.8% after Citigroup cut the investment company to neutral from buy.

Also on the decline, SABMiller PLC (SBMRY) eased 0.8% after Nomura cut the brewer to reduce from neutral.

As a sector, mining firms added the most pressure on the London index as metals prices mostly moved lower. Shares of Vedanta Resources PLC lost 1%, Randgold Resources Ltd. fell 2.6%, and Rio Tinto PLC (RIO) shaved off 0.6%.

Further of interest for investors in London, the U.K. government announced plans to take Royal Mail PLC public in "the coming weeks". The company will be listed on the London Stock Exchange and the minimum application offer for interested U.K. shareholders is GBP750 ($1,187).

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