The Boundary Commission for England worked "to the letter of the law" when planning its constituency boundary changes for the 2020 general election, according to Electoral Reform Society representative Will Brett.
The changes will see a number of MPs, including Labour leader Jeremy Corbyn, lose their seats.
Brett, the Head of Campaigns for the society, defended the Commission.
"To the letter of the law, there's nothing else they could have done," he told Julia Hartley-Brewer. "The problem is the letter of the law is not fair on voters.
"This is because this system is based on the Electoral Register, which is incomplete.
"One of the problems with the new system is it can be quite disconcerting for voters, who might not know what constituency they're in.
"The commission has a hell of a job to do in making this all work and trying to equalise the constituencies."