The FTSE 100 fell 12.2% on October 20, 1987, during the global stock-market crash.
The fall came one day after the United States experienced its record 22.6% one-day percentage decline in the Dow Jones Industrial Average. Financial markets around the world were closely connected through international investment, computerised trading, and rapidly moving news, so selling spread across time zones.
The FTSE 100 had already fallen sharply on October 19, when it dropped more than 10%. On October 20, the index suffered another exceptionally severe decline before recovering part of the loss. The crash affected shares in London and many other financial centres, although it did not produce a depression comparable to the 1930s.
Possible explanations included overvalued shares, rising interest rates, trade concerns, portfolio insurance, and automated or program trading. No single cause fully explains the worldwide speed and scale of the sell-off.