London's leading share index is down by nearly 100 points but it is FTSE 250-listed travel group Thomas Cook that is the main talking point after a profit warning.Thomas Cook's shares are down by more than a fifth after it said its third quarter results will miss expectations as the turmoil in the Arab world and economic weakness in the UK take its toll on the travel operator. The firm said its French operations have been hit particularly hard by the political unrest in countries such as Egypt, Tunisia and Morocco. Peer TUI Travel is also hurt.Oil outfit Premier Oil moved to assure the market about production targets saying it predicts a sharply increased production rate of around 60,000 barrels of oil equivalent per day by the end of the year, as its Asian development projects ramp up output.Information technology solutions provider Computacenter said profitability in the first half of its financial year will be comfortably ahead of the same period last year, and trading at this stage remains in line with management expectations for the year as a whole.Turning to smaller companies, scheduling and operations management software provider ServicePower Technologies is higher after saying it expects to report results ahead of expectations for the six months to 30 June after a busy period for the company. In economic news, the retail sales environment improved slightly in June, but conditions remain tough due to consumer caution and shops are having to launch sales to draw in customers, according to the British Retail Council (BRC). Total sales were up by 1.5% during June, having fallen by 2.1% in May. Investors in Yellow Pages publisher Yell have something to shout about after it announced a tie-up with operating system giant Microsoft to provide advertising for small and medium businesses. Yell said it would be teaming up with Microsoft to offer search, mobile and local advertising services to small and medium businesses to make the most of new business models arising from the growth of "cloud" computing. FTSE 100 - RisersShire Plc (SHP) 2,005.00p +0.15%British American Tobacco (BATS) 2,832.50p -0.25%Fresnillo (FRES) 1,430.00p -0.28%GlaxoSmithKline (GSK) 1,353.00p -0.33%Tesco (TSCO) 408.65p -0.47%Reckitt Benckiser Group (RB.) 3,474.00p -0.60%Imperial Tobacco Group (IMT) 2,126.00p -0.61%Inmarsat (ISAT) 561.00p -0.62%Hammerson (HMSO) 454.00p -0.63%Scottish & Southern Energy (SSE) 1,398.00p -0.64%FTSE 100 - FallersLloyds Banking Group (LLOY) 42.86p -4.43%ARM Holdings (ARM) 604.50p -3.67%Man Group (EMG) 243.00p -3.53%Barclays (BARC) 226.40p -3.23%Royal Bank of Scotland Group (RBS) 34.61p -3.11%GKN (GKN) 227.90p -3.10%Aviva (AV.) 407.00p -2.89%Whitbread (WTB) 1,551.00p -2.70%International Consolidated Airlines Group SA (IAG) 230.90p -2.66%Autonomy Corporation (AU.) 1,708.00p -2.51%FTSE 250 - RisersJD Sports Fashion (JD.) 1,045.00p +2.15%Unite Group (UTG) 215.00p +1.32%African Barrick Gold (ABG) 421.30p +0.45%UK Commercial Property Trust (UKCM) 80.95p +0.43%Impax Environmental Markets (IEM) 118.70p +0.17%Northumbrian Water Group (NWG) 447.80p +0.02%F&C Commercial Property Trust Ltd. (FCPT) 103.10p 0.00%Merchants Trust (MRCH) 423.80p 0.00%St. Modwen Properties (SMP) 189.60p 0.00%Mothercare (MTC) 420.90p 0.00%FTSE 250 - FallersThomas Cook Group (TCG) 91.55p -25.39%TUI Travel (TT.) 207.10p -6.37%Sports Direct International (SPD) 241.40p -5.70%Premier Foods (PFD) 17.07p -4.42%Redrow (RDW) 123.20p -4.12%Yule Catto & Co (YULC) 233.20p -4.03%Ferrexpo (FXPO) 447.00p -3.93%Premier Oil (PMO) 418.10p -3.84%Misys (MSY) 397.80p -3.82%Aquarius Platinum Ltd. (AQP) 300.50p -3.59%