A week which kicked off with dealers braced for turbulence in the wake of the collapse of Yamaichi, the Japanese broker, ended with no more than a whimper - until the last few minutes of trading in London.

Then the market's somnolence was given a sudden shake when the FTSE 100 index dropped 30 points only minutes before the close of trading, a move which stunned many dealers.

The sudden drop was variously ascribed by traders to a bout of frantic basket trading - arbitraging between the futures market and the underlying cash market - and over-the-counter (OTC) expiries, individually tailored options products engineered by one of the big US investment banks.

There were also claims that dubious prices were being entered into the order book. J P Morgan was one name put forward as behind the OTC story, which drove underlying share prices lower.

The damage caused by the OTC expiries was concentrated in a handful of stocks heavily influenced by US investors, notably SmithKline Beecham and Glaxo Wellcome, both of which fell between 6 and 8 per cent.

Most British shares drifted back to close with relatively light losses, lacking its usual lead from the US on Thursday, where Wall Street was closed for the Thanksgiving holiday.

Turnover eventually reached 584.1 million shares by the 6 p.m. cut-off point, of which 53.5 per cent was in non-Footsie stocks. The late burst of selling drove the FTSE 100 down 57.2 to 4,831.8, extending the fall on the week to 154.0.

Although London rallied strongly in midweek, traders cautioned that more bad news from the Far East could yet appear.

The FTSE Mid-250 was immune from the sell-off in the leaders, finishing 1.6 down at 4,656.7, down only nine points on the week.