Shares in IG Group fell more than 27 per cent after the owner of the IG Index spread-betting business said it would suffer a huge jump in doubtful debts. Its clients were caught owing significant sums after a plunge in Royal Bank of Scotland shares last month.

While the company benefited from extreme volatility last month, the week in which the UK government unveiled its bank bail-out plan saw dramatic falls in bank share prices, including RBS, which fell by half over two days.