Which day in 1987 produced the biggest one-day percentage fall in the FTSE 100?

20 October 1987 produced the biggest one-day percentage fall in the FTSE 100.

The British index dropped 26.45% on 20 October, one day after the U.S. Dow Jones Industrial Average suffered its record 22.6% percentage decline. Because time zones placed London’s session after the initial U.S. shock, the British market’s worst day occurred on a different calendar date.

The crash affected markets worldwide. In the United Kingdom, the fall reflected international contagion, large price movements in financial shares, and the broader reassessment of risk that followed Black Monday. The market later recovered much of its lost ground, but the event permanently influenced financial regulation and trading safeguards.

A frequent error is answering 19 October, the date associated with the Dow’s U.S. record. The same global crash produced different local worst days because exchanges opened and closed at different times.