The election watchdog has called for a review of the north's postal and proxy vote system after it emerged that almost three-quarters of rejected applications were due to issues with the digital registration number (DRN).
More than 5,000 applications for postal and proxy votes in May's council elections were turned down due to problems related to the DRN.
Applicants are required to provide a DRN when they register online to vote by post or by proxy – a requirement unique to Northern Ireland.
But 73 per cent of the total 7,017 rejected postal or proxy vote applications in May's council elections were not approved due to issues with the DRN.
These included people submitting their application without a DRN number or filling in the wrong number.
In May, former Belfast lord mayor Kate Nicholl said her postal vote application had been rejected due to the absence of a digital registration number.
The Alliance MLA, who planned to be overseas at the time of the council election, said her application was initially accepted by the Electoral Office and that "many constituents had also fallen foul of the situation".
The Electoral Office has said it cannot say what number of people were unable to vote as a result of not being able to vote by post or proxy, as they could have subsequently voted in person.
The figures were revealed in a Freedom of Information request from the BBC to the Electoral Office.
Interim chief electoral officer Sarah Ling said she was aware many voters had "struggled to understand the DRN process".
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She said a DRN had been required since 2018 but the need for people to re-register following a canvass in 2021 had exacerbated the issue.
"We are conducting a review to understand what more we can do to help electors through the process," she said.
"However, the legal requirement to provide one when someone has registered online is not within our remit to change."
A report last year from the Electoral Commission found the main reason for postal and proxy votes being rejected in the May 2022 Stormont election was due to the DRN not being provided.
This accounted for more than 60% of the total 6,031 rejected applications to vote by post or proxy.
The commission called on the British government to review the DRN process "to ensure it does not prevent people from accessing their vote".
