A minister has insisted the new postal voting system being used in Thursday's elections are "as secure as possible".
Postal voting rules have proved politically controversial
Constitutional affairs minister Bridget Prentice spoke after figures emerged suggesting postal fraud in 2004 might have been more widespread than thought.
More than 20,000 postal voters have dropped off the register in Birmingham wards investigated over fraud.
Mrs Prentice rejected criticism, saying the aim was to make polls secure and "as fair and as open... as possible".
A High Court judge said, when he concluded there had been vote-rigging in Birmingham's 2004 council elections, it would have "shamed a banana republic".
In Aston and Bordesley Green wards - which were the focus of the investigation - the number of postal voters this year is down by 80%.
In four other wards, where there were allegations of fraud at the time but no formal inquiry, more than half the postal voters have disappeared from the list.
Elsewhere in the city, the figures have remained about the same.
The numbers began to fall when West Midlands Police and the city council carried out an audit to check that existing voters knew they were registered.
They have continued to drop since the introduction of new computer checks.
Opportunity to vote
Mrs Prentice told the BBC she was confident that the new checks introduced this year - in which postal voters have to provide a signature and date of birth - were as "secure as possible".
She promised they would be reviewed after the elections "with the Electoral Commission" to see "if there's anything else that we need to do to make elections as fair, and as open, and as available to people as possible".
"One of my concerns, as well as fraud and making the vote as secure as possible, is also making sure that those people who are entitled to vote are actually on the register in order that they can vote," she added, on Radio 4's Today.
But election law expert Richard Price QC said individual registration - where any person going on to the Electoral Register must provide proof of identity as well as a signature - had "proved to be a complete success" in Northern Ireland and called for the government to "embrace" it.
That suggestion was rejected by Mrs Prentice, who said: "In Northern Ireland the register dropped very markedly after individual registration was brought in and it hasn't really gone back to the same figures again."
'Checks and balances'
Mr Price also raised concerns about electronic voting, which is being piloted in some areas and which he said "is open to serious abuse".
His concerns centred on the fact that pin numbers are posted to households, but "anybody can have access to them, people can collect them, go online and vote. There is no reliable method of checking".
But Mrs Prentice insisted that "because of all the checks and balances that are put into electronic voting that in some way it is an even more secure system than postal voting".
The issue of postal voting security has been the subject of political battles in recent years at a local and national level, over where the balance should be between designing out fraud while also making it easier for people to vote.
This year is the first time where people have to provide signatures on their postal votes, but ministers rejected calls for individual registration of voters rather than continuing the system where one person could apply for a postal ballot on behalf of a household.
There had been fears raised by some returning officers of possible problems with computer systems designed to verify the signatures on the postal ballots.
But the firm behind the system has said it is confident their systems will work.
'Mother of all democracies'
Conservative constitutional affairs spokesman Oliver Heald called for further research into the electoral register as there were "millions of people" on it who were not entitled to be there.
"It's wrong in the mother of all democracies that we shouldn't have a clean register that is accurate."
The Hansard Society, which promotes effective democracy, said it welcomed any moves to make voting more accessible, secure and confidential.
"The public must have faith in the political process that underpins our democracy," chief executive Fiona Booth said.