SHANGHAI shares fell yesterday in trading that lasted for only about 30 minutes after the opening because of a computer failure, with the index hitting its lowest level since 1993, brokers said. Brokers said after trading started at 9.30am the official composite index, which measures both the A and B share indexes, had dropped below the 720 mark. ''Big selling lots emerged after the market opened, giving it a heavy turnover of close to 300 million yuan (about HK$265.5 million). Investors are dumping their shares, and the index is falling,'' one broker said. Although the official explanation from the exchange said the computer breakdown caused trading to close earlier, brokers said there was a ''ghost'' at the exchange. He queried whether the computer had failed or had been made to fail, so that the index would not slide further. The exchange first announced trading would resume at 1 pm, then 2p, then 3.30 pm and finally said there would be no further trading for the day. When trading ended, the composite index had dropped by 7.79 points to 718.52. Figures of Credit Lyonnais Shanghai indexes were not available. It was not clear whether that was the final index. The broker said the increase in shares being traded in the stock market was to blame for the gloomy A share market, while the Sino-British impasse was causing B share trading to slide. Shanghai Hero Co yesterday announced that its A shares would be listed on March 11.