Hundreds of students and under-18s have been registered to vote without their consent following admin errors.
Plymouth City Council said it meant 635 students were registered by mistake and another 247 who are unable to vote were sent polling cards.
Tagging errors originating in May had caused both students and young people to be automatically added to the register, the council said.
It has since removed anyone who should not be on it, a statement added.
In the statement, Plymouth City Council said: "We thought we had found all the entries in our 192,000 records in May and deleted them.
"We have reviewed our register and found that we missed 635."
A further 215 over-18s who are not at university were also added to the register without their consent.
Errors also meant under-18s in the Plymouth Sutton and Devonport constituency were sent polling cards.
The information about students had been provided by universities and of those added without their permission, 137 had been sent a voting card.
The council said it was writing to all those affected to notify them they will need to register themselves if they wish to vote.
In a letter to the electoral commission, the conservative candidates for the three constituencies affected have called for an investigation into the incident and urged the council to remove the names of those incorrectly added to the register.
One of the signatories, Johnny Mercer, Conservative candidate for Plymouth Moor View, said he was "really disappointed" by the actions of Plymouth City Council.
"I want as many people to vote as possible, but don't believe in these practices which are inevitably open to abuse, and are not legal", he said.
Luke Pollard, Labour candidate for Plymouth Sutton and Devonport, said the city council needed to get its "house in order".
He said: "Let's get everyone registered and let the people cast their votes freely."
The Electoral Commission said it was "disappointed" by the reports after identifying a similar issue in May and said it would be working closely with the city council "to fully understand and address the issues that have been identified".
"It's critical voters can trust in our electoral system" said Ian Poyser, Green candidate for south-west Devon.
He said it was "extremely concerning" the city council had come under the spotlight "for the wrong reasons" for the second time and added: "These kind of errors erode that trust."
Sarah Martin, Liberal Democrat candidate for Plymouth Moor View, said she was "happy" to hear the mistakes were being rectified.
Other candidates have been contacted for comment.
Details of all of the candidates standing in all seats in Plymouth are available here.
What questions do you have about the local issues that matter to you in this election? Let us know by completing the form below.
In some cases your question will be published, displaying your name and location as you provide it, unless you state otherwise. Your contact details will never be published. Please ensure you have read the terms and conditions.
If you are reading this page on the BBC News app, you will need to visit the mobile version of the BBC website to submit your question on this topic.