The London Stock Exchange has placed trading restrictions on three unidentified brokerage firms that have not made sufficient progress in their backlog of unsettled trades. The restrictions weren't related to the London market's recent tumble, an exchange official stressed. The restrictions, which take effect Nov. 9, are based on the level of transactions that remained unsettled for one month, she said. The London Stock Exchange has told the three brokerage companies to restrict trading "by a certain amount," but it is up to the firms to decide how to limit their trading. The backlog stems from huge increase in the volume of trading since Big Bang, the year-old sweeping deregulation of the British financial markets.