The number of voters could drop for the first time in six years after nearly a quarter of a million voters put themselves at risk of disqualification for failing to verify their addresses.
It means that only 3.45 million Hongkongers will be eligible to vote in the Legislative Council elections in September.
A spokesman for the Registration and Electoral Office (REO) said yesterday its unprecedented clampdown on vote-rigging activities had left 230,000 voters on an omission list because they had so far ignored requests to validate their address, despite being given six weeks to reply.
Another 25,000 non-respondents were believed to have died.
However, a two-month voter registration campaign which ended last month enrolled 147,000 new voters, which means the total number of voters would fall by 108,000 - 3 per cent fewer than last year.
Voters on the omission list still have until the end of this month to respond before being disqualified.
Raymond Tam Chi-yuen, minister for constitutional and mainland affairs, said the size of the omission list did not surprise him. He said previous years had seen about 100,000 disqualifications and added: 'This year we have done more checking by random sampling and visits to housing estates.'
He said it was just one of their efforts to strengthen the electoral system against vote-rigging after November's district council elections were marred by accusations of people giving false addresses. Some addresses did not even exist.
The administration is keen to avoid a repeat of the scandal.
The REO said it had earlier tried to confirm addresses in different ways, including checking them against the records of the Housing Authority and checking on recently demolished and soon-to-be-demolished buildings to identify false addresses.
About 1.7 million voters were covered in the exercise and the office then sent 296,000 letters to voters whose addresses were in doubt and requested that they confirm the addresses were correct.
Only about 40,000 had replied by May 25. The office has referred cases involving about 1,500 voters to the police and 600 to the Independent Commission Against Corruption for investigation. At least 53 people have been arrested by the ICAC and 36 were charged on Wednesday.
In March, seven Hong Kong men living in Shenzhen were given suspended jail terms for using a false address. They were the first batch of defendants to be convicted and sentenced since November's polls.
While most people may simply not have been bothered to return their forms, Dixon Sing Ming, a political scientist at the University of Science and Technology, said it was possible that the jump in the number of ineligible voters exposed vote-rigging that could have existed for years.
He said he suspected some officials might have been aware of the problem but chose not to deal with it.
The number of voter cases referred to the ICAC by the Registration and Electoral Office. Another 1,500 were referred to the police