Hundreds of thousands of Scots eligible to vote but not registered have until next Monday to sign up.
Between 630,000 and 890,000 people in Scotland eligible to vote are believed to be missing from the electoral roll.
Electoral Reform Society Scotland (ERS) called for a push by public bodies and civic society to encourage them to register by the April 19 deadline.
Willie Sullivan, ERS Scotland senior director, 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 May’s elections will be on the basis of a flawed franchise.”
The campaign group has called for automatic registration to be introduced to ensure people do not lose out on the right to vote.
Sullivan said: “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.
“Parties must respond with action.
“Let’s ensure that the next election does not exclude huge swathes of our country and instead represents the gold standard for participation.”
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