Thousands of people across three Welsh villages have not received their polling cards ahead of Thursday's European elections.
Residents across Magor, Undy and Rogiet have been assured polling cards were sent out but have not been delivered by Royal Mail.
It is believed up to 10,000 people haven't received polling cards.
Ward councillor Frances Taylor said she has been told polling cards were sent out in the post on April 15 but they appear to have gone missing.
Royal Mail said it was urgently investigating the situation.
Cllr Taylor said: "It looks like there have been quite a lot of people affected. For people who have lived in the area for a long time they will know there's an election and where to go, but for people new to the area and for people who have not voted before they may not have all the information."
Residents have expressed concern over fears they wouldn't be able to vote.
Sheila Grosvenor, 59, from Magor, said she was worried she and her family had slipped off the electoral role.
She said: "I was concerned that my family were not eligible to vote or had somehow slipped off the electoral register as I had checked online in March and confirmed that we were registered to vote.
"I learned yesterday that there was a significant volume of other Magor and Undy residents who had not received their poll cards either.
"I am concerned that this could result in a significantly low turnout and misrepresentative result, as people may not realise they can still vote."
Returning officer Paul Matthews stressed that residents would be able to participate in the election, provided they are registered to vote.
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A spokesman for Royal Mail said: "We are urgently investigating the situation to find out what has happened.
"We will update as soon as we have more information."
