A glitch in a computer used in calculating the Dow Jones Industrial Average (DJIA) triggered a sudden drop in the blue chip index on Tuesday afternoon and added to the concern caused by Tuesday's stock sell-off.

The problem began at 1:50pm Tuesday amid heavy selling and caused a 70-minute time lag in calculating the value of the DJIA, according to a statement from Dow Jones Indexes. A system that feeds market data to the computer calculating the DJIA suddenly began experiencing delays. "While the DJIA was still being calculated and disseminated, the calculator was not receiving the underlying component prices of the DJIA on a timely basis."

When the problem was identified, Dow Jones Indexes switched over to a back-up computer and the result was a sudden 200-point plunge in the DJIA as the system caught up with the latest market data. "This switch-over caused prices that were received during the latency period to be processed all at once, bringing the index immediately in line with its underlying component stocks.

"Dow Jones Indexes is continuing to investigate the latency issue to correct the root cause of the problem."

Tuesday witnessed the biggest one-day slide of the New York Stock Exchange and S&P 500 since 11 September 11 2001. The Dow Jones Industrial Average lost more than 416 points in the sell off, which also hit some of US online brokerages.