Around 7.5 million people are not properly registered to vote in Great Britain.
The Electoral Commission said its latest research suggested the number had stabilised since 2011 after rising for a decade.
But with younger people the most likely to be missing, it said it hoped a new system of online registration will start to swing things back the other way.
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The commission said parliamentary registers were 85.9 per cent complete and that local government electoral registers 84.7 per cent - compared with 91 per cent and 92 per cent in 2002.