The FTSE 100 Index is nearing a four-month low at mid-morning, falling 46.8 points to 5026.5.
Sentiment is being hurt by falls in the US overnight, where the Dow Jones, Nasdaq and S&P all slid as investors took profits. A warning of lower revenues from fibre-optic maker Ciena also affected the mood.
Fallers in London include Vodafone, off more than 3% or 4½p at 127¼p; mmO2, down ¾p at 63p; and Cable & Wireless, off 9p at 205p.
Among the FTSE-250 stocks, Marconi is bearing the brunt of the depressed mood. Its share price is down 25% or 5p at 15½p - falling towards an all-time low closing price of 13p.
Colt Telecom is down 4p to 40p; Telewest Communications is off ½p at 18¼p; and Spirent has dipped 4½p to 127p.
Tech shares are also on the slide, with software firm Sage off 11½p at 198½p; and computer services group Logica down 9p at 406p.
FTSE-250 bus and train group Go-Ahead is down 55p to 532½p after reporting a slide in half-year operating profits and saying the downturn in aviation services and weakening rail usage would affect profits for the remainder of the financial year. Rival National Express is off 22½p at 525p.
Express Dairies is down 19% or 4½p at 19¼p after revealing it's no longer in takeover talks.