Hundreds of thousands of people in Scotland are not registered to vote in the event there is a snap general election, campaigners have warned.
The Electoral Commission estimates that between 630,000 and 890,000 people in Scotland were missing from the local government electoral roll in December.
Figures for the parliamentary register are not available, but campaign group the Electoral Reform Society (ERS) fears that the number of people missing from it is similar.
It is calling for automatic registration to be introduced across the UK to ensure people do not lose out on their right to vote.
Dr Jess Garland, director of policy and research for the ERS, said: "These figures should sound the alarm for anyone who cares about democracy.
"Hundreds of thousands of potential voters in Scotland are effectively missing from the electoral roll, representing a major barrier to political equality and democratic engagement.
"That means any snap election will be on the basis of an flawed franchise."
She went on: "You shouldn't have to opt in to your right to vote.
"As the Electoral Commission says, we need to move towards automatic registration now, starting with being able to check you are registered online, and being able to register whenever you engage with government bodies or services.
"There's widespread consensus on this - now it just needs to be done."
Electoral Commission analysis found that in December 2018, parliamentary registers in Scotland were 84% complete and 87% accurate while local government registers were 83% complete and 86% accurate.
It found there were between 400,000 and 745,000 inaccurate entries on the local government registers that month.
The ERS is also concerned that the number of people registered to vote in Scotland has dropped despite the population rising, and an increase in the number eligible to be on the register through the adoption of votes at 16 and 17.
Alice Kinghorn-Gray, campaigns officer for Electoral Reform Society Scotland, said: "That voter registration numbers are actually going down is deeply concerning.
"We urge the Scottish government to explore how it can use its powers to ensure everyone has a stake in our democracy.
"The gaps in registration are creating major inequalities in our elections, with young people and renters particularly affected."
A Scottish Government spokeswoman said: "It is important that everyone who is entitled to vote ensures they are registered to do so in time for any forthcoming election.
"The annual canvass of voters is currently under way - and every residential property in Scotland is being sent a letter requesting details of any resident who may be entitled to register.
"Anyone identified as not being on the electoral register will then be sent an invitation to register.
"Electoral registration officers produce interim registers in the run-up to any poll, but voters can register up to 12 working days before the day of any election."