Bid to promote voter registration
Around 7.5 million people are not properly registered to vote in Great Britain, a watchdog said.
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.
Under Individual Electoral Registration, in force now in England and Wales and Scotland after September's independence referendum, each person has to sign up individually instead of one person in a household supplying the details of all the people living at that address.
The Commission said parliamentary registers were 85.9% complete and that local government electoral registers 84.7% - compared with 91% and 92% in 2002.
Its research confirmed previous findings that those most likely not to be included are those who move house regularly - especially young renters.
There is also a large gap between the 85.9% of white and 83.7% of Asian people who are registered and the 76% of black residents.
"These findings provide an invaluable picture of the state of electoral registers before the change to Individual Electoral Registration," Commission chair Jenny Watson said.
"It's encouraging that registration numbers have stabilised since 2011, but nobody should underestimate the scale of the challenge in ensuring the registers become more complete."
"If you've moved house recently you're far more likely not to be registered and this has an impact on the groups affected, particularly younger people who tend to be more mobile.
"Online registration should make the system much more accessible and we're working with Electoral Registration Officers and a wide range of other organisations to get people registered ahead of the general election next year."
People can register at www.gov.uk/register-to-vote.
The Commission said it was examining how "selfie" photographs could be used to encourage people to use their vote.
It advised staff to stop people taking them in polling booths during May's local and European elections in case they inadvertently broke the law.
It s a criminal offence to reveal how someone has voted or will vote, as well as to communicate the unique serial number on a ballot paper.
In a report on those elections it said it would continue that advice.
"But we are also looking at how election selfies could be used to encourage voting in the 2015 polls and will issue guidance on this before these elections," it said.
It also called for the names of each parties' candidates in elections using a list system such as those for the European Parliament to be removed from the ballo paper in a bid to make it more manageable.
It received dozens of complaints in May about ballot papers having been handed to voters folded for convenience, meaning some parties may have been obscured.
Reforms were also needed to end the "unacceptable" exclusion of some EU citizens from casting their votes in the UK because of an over-complex registration system which requires them to complete multiple forms.