Nearly a fifth of voters are missing from the electoral register, according to research from the Electoral Commission.
The independent elections watchdog's report, funded by the cabinet office, shows at least six million people were not registered to vote at the end of 2010. The figure is rising with eight-and-a-half million people, 17.7% of the electorate, found to be unregistered in April 2011.
The report also finds just under half of 19-24 year olds are not registered to participate in elections, compared to 94% of those 65 and older.
In response, a Cabinet Office spokesperson said "far too many people" were not registered to vote. But the spokesperson emphasised that the government is "absolutely committed" to getting more on the electoral register.
"As part of this work, we are actively exploring ways in which we can make it as easy and secure as possible for citizens to register to vote, for example by enabling online registration."
Labour shadow justice secretary Sadiq Khan said it was a "disaster for British democracy", adding: “This report is a timely reminder of the need to work to increase electoral registration levels. It also highlights the fact that the Government’s flawed proposals for introducing individual electoral registration must include adequate checks and balances to mitigate against a sharp fall in the numbers registered to vote.”
Jenny Watson, Chair of the Electoral Commission, said anyone who cares about democracy would be concerned by the figures.
"There are many reasons behind the decline in registration – including changes to our population and increasing disengagement with traditional party politics. But we know almost half of those not registered mistakenly think they are, and more needs to be done to address this."