The FTSE 100 index has fallen 60 points to 5071.4 by lunchtime.
Shares are reacting to Tuesday night's slump on Wall Street.
Corporate worries hit US stocks, with concerns surrounding Enron's collapse and telecom firm Global Crossing's move for bankruptcy protection.
In London, Cable & Wireless slid 24¾p to 656½p, mmO2 was off 3¼p at 76¼p, Vodafone is down 3p at 151¼p and FTSE 250 operator Orange has slipped 15p to 510p.
Other FTSE 250 fallers are Energis, sliding nearly 4% or ¾p to 18¼p, Colt Telecom off 5¼p at 67½p and equipment firm Spirent down 7p at 156p.
Banks also weighed on the market with Abbey National down 25p at £10.25, HBOS off 14p at 822p, Royal Bank of Scotland down 34p at £17.59, Lloyds TSB off 12p at 748p and Standard Chartered 11p lower at 758p.
Northern Rock, which today reported a 18% rise in profits and buoyant lending, also fell, off 13p at 658p.
But with bumper consumer record figures released for December, retailers were having a good day on the market.
Next rose 15p to 937p, Marks & Spencer is up 2½p at 372½, GUS gained 3p to 629½p and jeweller Signet was up a penny at 99½p.
But it is a bad day for FTSE 350 antiques and paintings dealer Partridge Fine Arts. Shares slid 5%, off 3½p to 61½p, as the New Bond Street-based group said it had seen a slow start to the year.