RETROSPECTIVE legislation might be used to restrict cross-media ownership under plans being considered by the Government. Such a move might threaten the growing television and newspaper empire of Malaysian tycoon Robert Kuok Hock Nien, who has signed a deal to buy 34.9 per cent of the South China Morning Post, and Rupert Murdoch's News Corp, which would have 15.1 per cent of the Post if the Kuok deal went through and owns 64 per cent of STAR TV. ''Retrospective law is a sensitive issue, but clearly it is an option,'' the Secretary for Recreation and Culture, James So Yiu-cho, said yesterday. ''At the moment we are considering a cut-off level of 20 per cent ownership in each medium,'' he said. Legco would make a decision next April. The legal community said retrospective laws were potentially difficult. Dr Nihal Jayawickrama, a senior Hong Kong University law lecturer, said such laws could be ''disruptive''. ''It makes it difficult for people to plan their activities - it is only possible to plan according to existing legislation,'' he said. Roderick Woo Bun, the chairman of the Law Society, said a retrospective law in the case of cross-media ownership could be introduced because there was no previous law on cross-media ownership.