A polling station manager says problems with voter registration at the general election were unprecedented, in his experience.
The manager at a South Down polling station said dozens of people turned up with a polling card but were not on the register.
Because of the scale of the problems, he decided to allow affected people to vote.
The Northern Ireland Electoral Office was not contactable.
The polling station manager - who has worked in polling stations for more than 30 years - told BBC Radio Ulster's The Nolan Show that people were turning up with a polling card and the correct identification, but had been left off the register.
"I could see it happening once where there is an error made where you're not on the register, and you could be offered a tendered ballot paper," he said.
"But this was happening so often, rightly or wrongly I made the decision to give them a vote".
'Something has gone wrong'
He said all of the three ballot boxes in his station in Downpatrick had the same problem.
"It was happening, on average once an hour, I presume where people were coming in with the cards but were not on the register. I presume that's where the Electoral Office get their information from to send out the polling cards - from the electoral register."
Electoral law expert Stuart Wilks-Heeg from Liverpool University told the Nolan Show "it does sound like something has gone wrong" and suggested there should be an investigation.
He said that there was a tight deadline for anyone wanting to appeal any election results.
He questioned whether the 21 days to appeal rule should be extended, given the information that came to light on The Nolan Show.
Concerns were raised by a former employee at the Electoral Office last week. He claimed that members of the public were given inaccurate information about the deadlines for the submission of additional evidence for people registering to vote.
A member of the public has contacted the Nolan Show with an email from the Electoral Office which appears to back up the claims.
An email, sent to him by the Electoral Office on 2 December requesting additional information, states that "the closing date for evidence is 3rd December 2019".
The actual deadline was 4 December, and the whistle-blower had claimed that the public were not informed about this deadline, and many were wrongly told the deadline was 3 December.
He said this was "misleading" and prevented people being able to vote.
Last week, the Electoral Office told the Nolan Show that staff "advised the public to get their additional evidence to the Electoral Office by the 3rd December to ensure it was received in time for the legal determination deadline on 4th December. Every single piece of evidence received before midnight on the 4th December was processed".
Last week, the Chief Electoral Officer told the Nolan Show: "I commit to the public to check all processes during the post-election review to ensure continued improvement.
"These claims made my someone who was in our office for a few weeks about large numbers of people being denied their vote are not correct.
"As soon as I return I will move through all the complaints and allegations and questions; indeed we always review such material after an election in order to improve.
"I talk to parties, their administrative teams, journalists and the Electoral Commission all the time about how we can do better.
"We are determined to provide the best service to the people of Northern Ireland."
The Electoral Office added: "All applications received before the deadline were processed in time for every successful applicant to be on the register in time to vote on 12 December election."