Voting has been under way in by- elections to choose successors to six Birmingham Labour Councillors, banned after vote rigging last year.
The representatives for Aston and Bordesley Green were forced to step down after the Electoral Commissioner found evidence of postal ballot abuse.
One councillor was later cleared, on appeal, of corrupt practices.
Commissioner Richard Mawrey QC upheld allegations of postal fraud relating to the ballot of 10 June last year.
The results were declared void and a rerun ordered.
"The system is wide open to fraud and any would-be political fraudster knows that," Judge Mawrey said at the time.
He said evidence of "massive, systematic and organised fraud" in the campaign had made a mockery of the election and ruled that up to 1,500 votes had been cast fraudulently in the city.
The deputy high court judge said the system was "hopelessly insecure" and expressed regret recent warnings about the failings had been dismissed by the government as "scaremongering".
He criticised the government's insistence that the current postal voting system was working.
"Anybody who has sat through the case I have just tried and listened to evidence of electoral fraud that would disgrace a banana republic would find this statement surprising," he told the court.