Government plans to make people prove their identity before voting are "deeply flawed", the Electoral Reform Society (ERS) has warned.
The scheme, which is being piloted in Bromley, Gosport, Swindon, Watford and Woking for the local elections on 3 May, is designed to combat voter fraud.
But the ERS says the new system could be a "calculated effort" to make voting harder for some citizens.
Voter ID fraud was "incredibly rare", it said.
The claim comes after a leaked letter from the Equality and Human Rights Commission warned the change could have a "disproportionate impact" on voters from minority groups, who may not possess appropriate ID.
"With millions of people lacking the right photographic ID - and no government plans for a universal, free alternative - this can only mean another barrier for honest voters," said ERS chief executive Darren Hughes.
He added: "Mandatory voter ID is a sledgehammer to crack a nut. It's time for an evidence-based approach instead."
The ERS is part of a coalition of charities and campaign groups opposed to the plans, including Age UK, Stonewall, Liberty and the Salvation Army.
On Monday, Cabinet Office minister Chloe Smith told the House of Commons no one would need to buy new ID documents to be able to vote.
"We already ask that people prove who they are in order to claim benefits, to rent a car or even to collect a parcel from the Post Office, so this is a proportionate and reasonable approach," Ms Smith told MPs.
"Democracy is precious and it is right to take that more robust approach to protect the integrity of the electoral process."