STOCK Exchange of Hong Kong chairman Charles Lee says the view of HSBC chairman Sir William Purves that the exchange had the potential to become the world's biggest within 25 years is entirely possible. Mr Lee told an Asian Bankers Association seminar yesterday that this scenario could not be discounted given the fact Hong Kong's stock market had jumped more than 400 per cent in the past four years as China's economic reforms fuelled tremendous growth. Sir William said by 2018 turnover on the Hong Kong stock market could roar past that of Tokyo and its capitalisation might exceed all other markets in the world. Mr Lee was joined at the seminar by International Securities Consultancy director Susan Selwyn, who said mainland officials had strongly rejected speculation that A and B shares would be combined into a single class. Ms Selwyn said she was told six weeks ago by China Securities Regulatory Committee officials that reports in Hong Kong newspapers about this development were ''a lot of rubbish''. Ms Selwyn, who served as deputy chief executive at the Securities and Futures Exchange from 1987 to 1991, said mainland officials made it clear the two distinct classes of shares would not be abolished. Stock Exchange of Hong Kong deputy head of listing Kenneth Koo said the introduction of H shares issued by mainland enterprises on the Hong Kong exchange did not detract from the fledgling markets in Shenzhen and Shanghai. ''Shenzhen and Shanghai will learn from us,'' he said. ''I don't view it as counter-productive.'' Mr Koo said investors, who criticised the lack of liquidity in China's B-share market, should remember B shares offer exposure to different size firms while H shares were issued by large companies. He said Beijing was pleased with investors' enthusiastic response to the six mainland enterprises listed on the exchange so far and discussions were now underway about a second batch of H-share candidates. Investors have complained recently about the refusal of Tsingtao Brewery and Guangzhou Shipyard to issue interim reports. However, Mr Koo said many newly-listed companies had not been required to file interims and it would be a double standard to require H-companies to do something different from Hong Kong companies. ''It's a tempest in a teapot,'' he said, adding all the H-share companies issued profit forecasts for the full fiscal year before listing.