By Alex Stevenson Follow @alex__stevenson
The number of people not registered to vote has more than doubled in the last ten years, research from the elections watchdog has revealed.
Around 8.5 million people were unregistered by this year's local elections, compared to 3.9 million at the time of the autumn 2000 canvass.
That equates to 17.7% of the eligible electorate not being included on the parliamentary register for this May's local elections and electoral reform referendum.
"This should be of concern for everyone who cares about democracy," Electoral Commission chair Jenny Watson said.
"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."
The dramatic increase in the number of voters who are effectively disenfranchised has sparked concern from critics of the government's plans to switch to a new system of voter registration.
Electoral Reform Society chief executive Katie Ghose said the report was "sobering for any democrat" but added: "Sadly that's just the tip of the iceberg, because under the government’s plans for voter registration that number is set to sky rocket.
"The problem isn't the principle of individual electoral registration - it's the reckless way it's being introduced. It will mean that three million people could be disenfranchised simply for moving home."
The report found that two to three million people across Great Britain may have moved house between December 2013 and July 2014, meaning they would not receive the form to register individually under the new system.
Ministers are insisting new techniques like data matching will help improve coverage, but experts say the jury is still out on whether these methods will mitigate the impact.
"The idea we're fighting general elections with seven, eight million people missing from the registers is extremely significant and worrying," Stuart Wilks-Heeg of Democratic Audit told politics.co.uk.
"We had no idea until this report that it was potentially that bad. It's a real wake-up call."
The Electoral Commission is calling on the government to hold an annual canvass in early 2014 to minimise the impact of the transition from household to individual voter registration.