New vote-checking technology may struggle to cope during next month's local elections in England and Wales, it has been claimed.
The postal vote checks could delay counting
Returning officers said postal votes could be thrown out because systems set up to tackle electoral fraud were not working properly.
Postal voters have had to supply their signature and date of birth.
Computers will scan and check the information, but one expert said the technology might not perform.
Malcolm Dumper, policy director at the Association of Electoral Administrators - which represents election staff - said officials were also concerned that legitimate voters could have their ballot unfairly rejected if their signature was not consistent.
About half of the English councils holding elections have said the checks will take so long that they will not start counting votes until the morning after the polls close, said the BBC's Ross Hawkins.
A spokesman for the Department for Constitutional Affairs postal votes would be counted, even if there were computer problems.
Russell Osborne of Northgate Information Solutions said: "We are confident that all of the 75 local authorities that we are working with will be able to successfully electronically validate their postal votes using the electoral administration software.
"However, there is also a robust semi-automated solution in place as a contingency, which will help ensure the smooth running of the elections in May and prevent unnecessary delays.
"Any form of innovation and improvement is likely to experience its share of teething-problems and naysayers, but we believe that the progress that has already been made with postal voting can help to deliver a modernised electoral management system, which will encourage greater participation and confidence in the democratic process."
Earlier this week, concerns were also raised about the counting of votes in the Scottish Parliament and council elections.
It is expected to be the UK's biggest test yet of an electronic vote counting system.
But experts from the US told BBC Scotland that the Scottish system lacked sufficient independent safeguards.
Deputy Scottish Secretary David Cairns said the system had been thoroughly tested and a hand recount was still an option if things went wrong.