Many people may find themselves unable to vote come polling day
More than half of young people eligible to vote are not registered to do so, the elections watchdog has warned.
Officials have been urged to make "one final push" to get more people registered for the general election.
The Electoral Commission said more than 3.5m people may not be registered. Its research suggested 56% of 17-25 year olds were not on the electoral roll.
Separately, the Hansard Society said its research suggested only 54% of people were "certain" to vote.
It said participation among young, working class voters was likely to be especially low - with less than one in four saying they would definitely vote - signalling their "mistrust" of politicians.
The Electoral Commission carried out research in eight local authorities representing urban and rural areas to establish how complete their electoral registers were.
People must be on the local electoral roll if they want to vote in the upcoming general election - widely expected to be held on 6 May.
HOW TO REGISTER TO VOTE
To vote in a general election your name must be on the electoral register
Check with your local council to find out whether you are on it
If you are not registered, you need to fill out and return a registration form
You can pick one up at your local election office or download one from the Electoral Commission website
The Commission said its findings suggested that 56% of 17 to 24-year-olds were not registered to vote, nor were 31% of voters from ethnic minorities. Of people who had moved house in the last year, only 21% were registered.
People can register to vote from the age of 16, although they will not be able to actually vote until they are 18.
The watchdog said while London had always had low rates of people registered to vote, that "decline" may be spreading to other metropolitan areas.
It cannot estimate the total number of people missing from electoral rolls until the next census is carried out in 2011.
But it said 3.5m eligible voters were missing in 2001 and due to a decline in registered voters in the early 2000s, there might be even more now.
In its annual assessment of the performance of returning officers, the Commission says generally they have improved across all areas.
But it said performance in raising public awareness of the need to register to vote was "particularly weak".
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"In many instances promotional work appears to be a token approach to demonstrate that the statutory duty is being met, rather than reflecting a well-thought-out approach to improving registration rates in localities where they are lower," the report said.
Commission chairman Jenny Watson said the general election could be called at any time and there may only be a few days to register once that happened.
She said the failure of some election officials to promote registration "isn't good enough" adding: "We have asked every registration officer to make one final push before the general election."
She said more "co-ordination and leadership" was needed among the 379 local registration officers operating across Great Britain.
"And we need to ask fundamental questions about the timing of our annual canvass in the autumn when elections are typically in the spring," she added.
Publishing its annual audit of political engagement, the Hansard Society - which seeks to strengthen parliamentary democracy - said politicians faced a challenge to close the gap between the number of people who felt a "duty to vote" and those who actually did so.
A survey of more than 1,000 adults conducted for the charity by Ipsos Mori found that while 76% of people questioned felt they had a duty to vote, only 54% said they were absolutely certain to do so.
It said part of the problem was fewer people considered Parliament "relevant" to their lives, with only 19% of people listing it as one of the most influential institutions in the UK - down 11% from 2004.
The Westminster expenses row had confirmed peoples' already sceptical attitudes towards politicians rather than substantially further eroding trust, it said.
It also highlighted what it said was a difference between how the public viewed politicians in general and the work of individual MPs, with the latter being viewed much more favourably.
"The public have long been sceptical about the motives of politicians and the expenses situation has merely confirmed their views," said Ruth Fox, director of its parliament and government programme.
"But the fact that the public now perceive Parliament to be a less relevant institution than previously is a worrying development that the new intake of MPs after the election must address."