Shares in the FTSE 100 Index dropped exceptionally by about 1.1% on Friday due to an unusually large trade by an investor.
The London Stock Exchange was forced to halt continuous trading for five minutes and hold an auction after the so-called "basket trade" of about 101 stocks.
The trade caused a 60-point or 1.1% drop in the Footsie, with eight or nine stocks particularly badly hit.
They included British American Tobacco (LON:BATS), Diageo (LON:DGE), HSBC (LON:HSBA), National Grid (LON:NG.), BHP Billiton (LON:BLT), Vodafone (LON:VOD), BP (LON:BP.) and Rio Tinto (LON:RIO).
Such an event happens when stocks rise or fall by more than a certain percentage threshold, which is in place to prevent extreme volatility.
An LSE spokesman said no trades had been cancelled and the system had returned to normal. He said the LSE was investigating the matter.