Data matching of voters has been declared a success after a "confirmation live run" over the summer - but some councils were "affected by IT problems".
The Electoral Commission watchdog revealed that the details of 87% of current electors have been transferred automatically to new registers - a higher proportion than expected. It means almost 37m people in England and Wales need take no further action ahead of the switch to individual electoral registration (IER) at the end of 2015, or a year later.
Of those transferred, 33.7m names were matched against the Department for Work and Pensions (DWP) database and a further 3.2m against local records, by town hall officers.
However, the details of 5.5m existing electors - or 13 per cent of the total - have not been matched or transferred, the commission revealed. It said that 329 of the 348 local authority electoral registration officers (EROs) in England and Wales had carried out matching so far, without identifying the 19 that have not.
It added: "Some EROs have reported being affected by IT problems which have resulted in them delaying the start of their write-out processes. The commission does not believe these issues will have any long term impact on the successful delivery of IER, but we will continue to monitor those software issues that are still outstanding. If at any time we believe that the software issues are such that there is a fundamental risk to the effective delivery of electoral registration services, we will say so."
Nevertheless, Jenny Watson, the commission's chairwoman, delivered an upbeat message, saying: "It's encouraging that 87% of people across England and Wales have been moved onto the new electoral registers.
"But there's still more work to do. Every electoral registration officer has detailed plans in place to reach those residents they were not able to transfer automatically.
"No one will come off the electoral register because of this change before the 2015 UK Parliamentary General Election."
Anyone who was not able to be transferred has been sent a letter asking them to provide additional information - their National Insurance number and date of birth.
Private renters, young people and students are least likely to be transferred automatically and will now be targeted by EROs.
Next summer, the commission will recommend whether IER should start in December 2015 - or be held back for a further 12 months, if voters risk being disenfranchised.
IER - which replaces the household registration system - is described as the biggest change to the electoral registration system in almost 100 years.
In addition to the electors transferred automatically, around 1.8m applications to the new register have been made via the online system since its launch in June, ministers say.
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