One in five illegal immigrants jailed during the past three years were repeat offenders, sparking calls for a re-think of custodial sentences. Correctional Services Department statistics show the number of illegal immigrants jailed increased from 2,756 in 1997 to 2,860 in 1998 and 3,107 last year. About 19 per cent had been jailed for other offences in Hong Kong. The figures represent only a small portion of double-backers - mainlanders who come across the border more than once. First-time illegal immigrants are usually repatriated immediately. Another set of data showed the proportion of illegal immigrants arrested for committing crimes other than immigration offences increased from 8.3 per cent in 1995 to 9.5 per cent last year. Illegal immigrants have accounted for about 25 per cent of the prison population over the past five years. The average population last year was 10,364, 19 per cent above capacity. Some illegal immigrant inmates are suspected to have broken prison rules to have their stay extended as they can earn more in prison than on the mainland. The chairman of the Correctional Services Officers' Association, Wong Wai-hung, said there were two incidents last year in which colleagues were attacked by illegal immigrants apparently seeking a longer sentence. 'They attacked the officers all of a sudden for no reason,' Mr Wong said. He said prisoners were now given alternative punishments such as isolation. Dr Roderick Broadhurst from Hong Kong University's centre for criminology said that it was difficult to say whether the increase in illegal immigrant inmates meant prison lacked a deterrent effect. He said the increase in the percentage was not significant enough to prove a trend and the number of illegal immigrants arrested could be affected by various factors such as their exposure to crime. But he said the Government should explore other ideas such as renting jail space on the mainland to imprison illegal immigrants.