SERIOUSLY handicapped people have been unable to register as voters because the identity document they hold does not fit the computer systems catering for normal identity cards, an organisation for the disabled said. Peter Chan Fuk-shing, chairman of Rehabilitation Alliance Hong Kong, yesterday told the Boundary and Election Commission that he had received one complaint in June from a patient who was rejected because he held a ''certificate of exemption''. The certificate is an identity document issued by the Immigration Department for the seriously handicapped. Although the document enjoyed the same legal status as normal ID cards, the pattern of numbers on it was different, so they could not be included on the Registration and Electoral Office's computer system, Mr Chan said. Mr Chan estimated that about 20 per cent of those who lived in rehabilitation centres held certificates of exemption, but he did not have the exact figure. He said it was intolerable that their rights were denied because of technical problems. ''They are Hong Kong permanent residents but unfortunately their political rights are seriously discriminated against by these minor problems,'' he said. Commission chairman Mr Justice Woo Kwok-hing promised to discuss the matter with the Immigration Department. But a department spokesman said the coding of the document would not be modified for the sake of convenience in polling. Nevertheless, Mr Justice Woo assured Mr Chan no mentally or physically handicapped people would be rejected for voter registration or voting provided he had a valid identity card. Mr Chan said some registration officers were hesitant to accept applications from the disabled.