There has been a large increase in applications for postal votes, research by the BBC suggests, which could add to fears of long delays in counting them.
Checking ballots could take days, if there are computer problems
Requests are up 550% in Middlesbrough, 400% in Fife and Lanarkshire, 600% in Flintshire and more than 200% in Luton and Somerset, the survey found.
Returning officers' spokesman David Monks said a last-minute rush and any computer problem could cause delays.
But those behind the software say they are confident it will hold up on 3 May.
Computers are being used for the first time in this year's elections to check the signatures of those voting by post, as part of an attempted crackdown on fraud.
But some of the equipment is still being tested and staff are still being trained - leading some to doubt the software could cope even without the surge in postal applications.
The survey by BBC Radio 4's World at One programme suggests there has been a huge increase in demand for postal votes.
Mr Monks, who speaks on behalf of English councils on election matters, said a last-minute rush of postal ballots, combined with any problems with the computer signature checking system, could cause problems for some councils.
He told the BBC: "The fallback position is manual checking and that will take a long time.
"If you have got to check that manually, we would need extra staff, they would need to be trained - that could take days.
"Certainly in those places where they have had a big increase, the systems will be thoroughly, thoroughly tested, so we hope they can cope with these new increases in numbers."
Burnley Borough Council, has already put plans in place to read thousands of signatures manually on 3 May - after problems with computer programmes which read voters' signatures.
Extra checks have been introduced following a series of allegations of vote rigging, since the UK introduced postal and proxy voting on demand in 2001.
A judge looking into vote rigging in Birmingham's 2004 local elections said he had heard evidence of fraud that "would disgrace a banana republic".
But Russell Osborne, whos company is supplying software to 75 authorities, said he was confident they would all be able to check their postal votes electronically.
"Any form of innovation and improvement is likely to experience its share of teething-problems and naysayers," he said.
"But we believe that the progress that has already been made with postal voting can help to deliver a modernised electoral management system."