The 1987 crash known as Black Monday caused the largest one-day percentage decline in the FTSE 100 Index.
On October 19, 1987, the FTSE 100 fell 26.45 percent, a decline even larger than the 22.6 percent fall recorded by the Dow Jones Industrial Average in the United States. The British market was part of a synchronized international sell-off that also affected Hong Kong, Australia, Canada, and other financial centers.
The collapse followed a period of strong share-price growth and rising concern about interest rates, currency values, and trade deficits. Program trading and portfolio-insurance strategies may have accelerated the selling, but historians and market regulators do not identify one simple cause. In Britain, the event is also remembered as a defining financial shock of the 1980s. Trading safeguards introduced later were intended to slow similar feedback loops.