Students who registered to vote say they were not told their applications had missing information – such as room numbers in addresses – which has made them “invalid”.
Cardiff Council said it had been unable to contact around 200 people who supplied incomplete addresses in their applications, to ensure they were registered in time for Thursday.
Many of those who will now be unable to vote are young people living in student accommodation blocks in Cardiff.
The National Union of Students (NUS) is calling on the council to place affected students on the electoral register before tomorrow, adding that any other outcome would be “unacceptable”.
Morgan Cullen, a student at the University of South Wales, said she only found out two days ago that her application was invalid after she contacted the council.
The 19-year-old typed in her postcode to the registration website and selected her address after it automatically appeared. She said she was not told that the application was missing information.
Ms Cullen, who lives in Liberty Bridge accommodation, where a number of students have been affected, told The Independent: “I didn’t even realise it was an issue until another student living in my accommodation said she had the same problem.
“I’m absolutely gutted, I’ve made such an effort to get involved with politics the past couple of months.”
The second-year student, who will not be able to vote, added: “I’m so aware of how crucial my vote is as a young person, which makes it so much more frustrating, especially since it wasn’t an error on my part.
“The system has completely let me down.”
Rob Simkins, president of NUS Wales, said: “I’m alarmed that a significant number of students may be denied the right to vote because of an administrative error that was no fault of their own. This is the general election that will define our futures and students are demanding that their voices be heard.
“Cardiff Council must now work with the accommodation provider and the students affected to make sure they are on the electoral register come Thursday. Any other outcome would be unacceptable.”
A spokesperson for Cardiff Council said: “The national electoral registration gov.uk website allows electors to provide addresses manually when registering to vote and unfortunately, a number of people living in student accommodation have supplied incomplete addresses.
“Without the full and accurate address, including flat and room numbers, it is not possible to add individuals to the register. Everyone registering on the site receives an auto confirmation message that the application has been sent to us, regardless if the information supplied is complete or not.
“It is the responsibility of the elector to validly register and there is no obligation on the Electoral Registration Officer to chase up invalid applications.
“However, considerable time and effort has been given to trying to contact those affected and of approximately 1,000 invalid applications, around 800 people have been contacted and registered.
“Many have not responded, others have not provided contact details and unfortunately, due to the high volume of queries currently being dealt with, we have not been able to contact all of those who made invalid applications.”