The FTSE-100 Index was down 29.1 points at 5205.1 after an hour's trading, reversing an earlier gain of 7.6 points.
The fall continues Friday's trend, when the FTSE closed down 33.9 points, while the Dow Jones Industrial Average in New York ended the week up only marginally.
Financial Spreads trader Mr Tom Hougaard blamed the slow start to the week on a lack of corporate news.
He said: "We are expecting a quiet day in the US. Its a pretty big week for UK earnings and economic data but, this morning, there is nothing going on. The Footsie is treading water - no one is eager to buy or sell, and we have seen hardly any business this morning".
He said the US would be quieter because the bond market was closed for the Federal Veteran's Day Holiday.
In London, banking and media stocks weighed on the market, with HSBC off 15p at 777½p, Royal Bank of Scotland down 29p at £16.41, Lloyds TSB off 11p at 702p, HBoS down 13p at 817p, and Abbey National 16p lower at £10.24.
Among the media fallers were Pearson, off 29p at 851p; Daily Mail & General Trust, off 19½p at 700½p; and Granada, down ¾p at 151¼p.
But Cable & Wireless was up 7¾p at 337¾p following speculation it could return about £1.5 billion to shareholders. The group will announce its half-year results on Wednesday.
PA