A rule in the United Kingdom that permits market makers to trade outside quoted ranges, when an exchange determines that market movements are so sharp that quotes cannot be kept current. The purpose of the fast market rule is to maintain an orderly market during a time of chaos. Under the rule, market makers must turn off their computerized trading systems, called black boxes. They do not have to quote share prices based on the London Stock Exchange's screen prices while the fast market is in effect, but they are still required to make firm quotes.
Fast markets are rare and are triggered by highly unusual circumstances. For example, the London Stock Exchange declared a fast market on July 7, 2005, after the city experienced a terrorist attack. Share prices were falling dramatically and trading was exceptionally heavy.
The fast market rule is made possible because circuit breakers are not used. Circuit breakers allow exchanges to temporarily halt trading during sharp price declines to prevent panic selling. With a circuit breaker, the sharper the decline, the longer trading is halted.