That means the inmates are bearing the full brunt of the tax rise which is made worse, they say, by the fact that cigarettes are an essential currency in the prison black market.

According to the Correctional Services Department, 80 per cent of inmates smoke, and they spend 60 per cent of their jail wages on tobacco.

'Cigarettes are like gold in prison,' a 50-year-old former inmate Ah Shing said. 'We not only smoke cigarettes, but we buy most things with cigarettes, too.'

In March, inmates' wages - which had been about HK$300 a month - were raised 7.26 per cent in line with higher food prices but this did not reflect the tax increase on cigarettes imposed the previous month.

Ng Wai-tung, from the Society for Community Organisation, said each prisoner had access to a one-off quit-smoking treatment but it was not effective. Ng, who helped former inmates lodge complaints to the Legislative Council about the prison wage policy, said he supported the government's effort to discourage smoking but it should introduce better treatment to help prisoners stop smoking, not make cigarettes harder to buy.

Lawmaker Leung Yiu-chung said: 'The policy will not combat prison smoking, but actually worsen the problem of prison gambling.'