Shortly after midday, the benchmark FTSE 100 share index was down 39.97 points at 7,444.13.

Retail distribution group Bunzl was the biggest faller in the index, down more than 6%, after Morgan Stanley flagged up the risk to the company from Amazon's business-to-business venture.

Burberry shares fell again, dropping nearly 4%.

On Thursday, shares in the luxury fashion group had sunk 10% after it unveiled plans to move further upmarket.

Marks and Spencer fell 1.6% and Primark owner AB Foods was down 1.8% as investors digested a week of downbeat retail news.

In the FTSE 250, Ultra Electronics fell more than 7% after the defence firm said a decision on its purchase of US firm Sparton was set to be delayed.

Ultra Electronics announced in July it would buy Sparton, which makes anti-submarine devices, for $234m.

However, it said in September that the US Department of Justice had asked for more information, and it now expects the department to make a decision at the end of March.

On the currency markets, the pound was up 0.3% against the dollar at $1.3182 and was also 0.3% higher against the euro at €1.1324.