But the scale of the potential problem with this year's elections has prompted anti-graft chiefs to draft in an extra 20 investigators.

ICAC commissioner Timothy Tong Hin-ming said the 20 new officers would start work today with 40 officers from Team X involved in the original probe.

Tong said: 'This is an extra. It is not to replace, but to intensify work by our present teams.'

The commission has received 421 complaints related to the district council polls. Of these, 174 involve corruption, including 27 of vote-rigging, and 211 publishing false information about candidates.

In the 2007 polls more than 1,200 complaints of all kinds were received by the anti-graft agency, but investigators believe that figure will be topped this year. Yesterday, the Civic Party submitted 496 addresses to the ICAC which they suspect are linked to vote-rigging.

At least four voters, each with a different surname, were registered as living at each address. The addresses span 32 constituencies and involve 2,430 voters. The poll attracted a record turnout of 1,202,544.

The Democratic Party has submitted more than 400 cases of suspected vote-rigging to the ICAC.

Civic Party Chairman Alan Leong Kah-kit said if the ICAC investigation found corruption or illegal conduct was prevalent, there would be grounds for a legal challenge to the legitimacy of the elections.

'The timing is quite critical. I hope they can complete the investigation of the more serious cases within two to three weeks so as to allow enough time for any election petition to be considered [which should be brought before January 5],' he said.

So far, 23 people have been arrested by the ICAC for alleged vote-rigging in the King's Park constituency of Yau Tsim Mong district. Seven were charged and appeared in court.

In addition, eight registered voters have been granted bail after being arrested by police in connection with vote-rigging in the Mei Foo South constituency in Sham Shui Po.

The government is considering asking newly registered voters, or those seeking to change their official residence, to provide proof of their addresses.

Secretary for Constitutional and Mainland Affairs Raymond Tam Chi-yuen said the government would also conduct random checks on its register of electors.

'These proposed measures will inevitably put some people off registering because it will cause them disturbance,' he said.

'However, I believe the public will understand that we are maintaining the fairness and authoritativeness of electors' registration.'

1,200

The number of complaints after the district council elections in 2007. Investigators believe that figure is likely to be topped this year