The Shanghai Stock Exchange plans to put price limits on pre-opening trading of some stocks, funds and bonds, a move brokers say will help protect retail investors from volatile opening prices. The new cap, which would require brokers to input stock prices during the pre-trading session, would prevent stocks from falling as much as 50 per cent of the previous close or rising up to nine times, the exchange said on its website. It did not say when the cap would begin. Prices of funds and bonds would not be allowed to increase more than 150 per cent or fall to 70 per cent of their last traded prices. The proposal was likely intended to protect investors from being fooled by substantial price movements during the pre-trading session, said Christopher Cheung Wah-fung, chairman of the Hong Kong Stockbrokers Association. 'Many firms clarify rumours or news reports close to the market opening,' Mr Cheung said. 'So some people may use the pre-opening session to manipulate share prices.' But Joseph Tong Tang, an executive director at Sun Hung Kai Financial, said the limit was too relaxed to have a big impact.