Tokyo Stock Exchange Inc., operator of one the world’s largest stock markets, is likely to face further scrutiny over the state of its IT systems after it was forced to halt trading 20 minutes earlier than normal on Wednesday because its computer system was close to capacity.

The trading halt at 2:40 p.m., local time, came after the exchange warned during the market’s lunchtime break that trading would end early should volumes reach 4 million during the afternoon session. At lunchtime, the number of trades had already reached 2.32 million and they hit 4 million at 2:25 p.m. The system is designed to handle 4.5 million trades per day.

The heavy volume came on the back of fallout from allegations of wrongdoing at Livedoor Co. Ltd., a major domestic Internet portal, and poor results from Intel Corp. and Yahoo Inc. The benchmark Nikkei 225 index closed down 3 per cent at 15,341 points, which was its biggest one-day fall in almost a year.

The shutdown is the latest in a string of IT systems-related problems to have hit the exchange in recent months.

In December the bourse’s software was called into question after an erroneous order to sell 610,000 shares of J-Com Co. Ltd., a newly listed company, for