Hundreds of local authorities may have failed to match data on voters, Labour has warned - as it raised fresh fears about the switch to individual electoral registration (IER).
The opposition said more than 250 local authorities have not "confirmed" whether they matched their electoral registers with central government databases. And they raised the alarm over almost 100 councils who have failed to conduct a proper door-to-door canvass in the past five years, to find 'missing' voters.
Stephen Twigg, Labour's constitutional affairs spokesman, pointed to fresh research by the Electoral Commission revealing that 7.5 million people - nearly one sixth of the electorate - are not registered. He said: "If you thought this was bad, it could get a whole lot worse as the government fast-tracks plans to introduce individual electoral registration.
"Although this does help automatically transfer a large bulk of the electorate, a further 8.7 million voters are in danger of losing their vote unless they register individually."
But Greg Clark, the outgoing Cabinet Office minister, insisted that Labour's information was "out of date".
He said: "Of the applications made since June 10, more than 90% have been successfully confirmed with Government data, so it is going extremely well. The electoral registration community around the country is pretty pleased with the progress."
Labour said later that its information came from a written parliamentary answer - in March - which stated that only 137 local authorities had matched data in last year's "dry run".
Among the big councils missing from the list provided were Leeds, Manchester, Liverpool, Bradford, Cardiff, Newcastle, Bristol and Hull.
In the answer, Tory MP Gary Streeter, a spokesman for the Electoral Commission Committee, stated: "Those that did not provide data may still have carried out local data matching work."
The Electoral Commission gave the go-ahead to the switch to IER last autumn, after declaring the dry run to have been a success.
The watchdog said the names and addresses of 78.1% of voters were matched to the Department for Work and Pensions (DWP) database - a higher rate than predicted.
However, the transition is being phased in over two years, which means no one registered to vote at the last canvass will lose their right to vote at next May's general election.
Meanwhile, electoral registration officers will write to every household to find missing voters, followed up with a door-to-door canvass - which will be assessed by the Electoral Commission.
But Labour also highlighted how 100 councils had failed to carry out adequate house-to-house enquiries over the past five years - with 35 failing the commission's test more than once. Topping that list is West Devon (five failures), followed by East Hertfordshire, Mid Devon and West Somerset (all four times).