Cornwall Council has had to spend almost £200,000 on chasing up households which have yet to complete their electoral registration forms.
More than 29,000 homes have yet to return their registration forms and the deadline is just days away.
Cornwall Council is required to produce an up-to-date list of voters annually which then stays in force for the year.
The new electoral roll is due to be published in December and all forms need to be returned by 24 November.
Registration forms were posted to all 253,362 homes at the end of August, but 12% have yet to be returned.
As a result of the response rate, which the council has described as "disappointingly low", the local authority says it has had to spend almost £200,000 of taxpayers' money sending out reminder letters and using teams of canvassers to help people complete the forms.
Cornwall's Acting Electoral Services Manager Paul Hamill said: "We are required by law to send out the reminder letters and door-to-door canvassers.
"We had hoped to have a good response to our original campaign in August and it is very disappointing that we have had to spend money which could and should have been spent on other council services."
Thirteen wards, including Cury, Mullion, Goonhavern, the Pentewan Valley and Sheviock, have already achieved a 100% response rate.
However, the council said that the response rate in Penzance east so far was just 49%.
Other low response areas include Camborne West, St Austell Gover and Newquay Rialton South, which have response rates of 64%, 67% and 73% respectively.