Papers are folded to protect secrecy, says the Electoral Commission
The elections watchdog has asked polling staff not to fold European election ballot papers, amid complaints part of the list was hidden from view.
The Electoral Commission sent out an alert after UKIP said parties at the bottom of the alphabetical list were hidden when the first fold was opened.
A number of voters, mainly from the Yorkshire and the Humber region, have contacted the BBC about the problem.
Voting for the European elections continues until 2200 BST.
Folded for secrecy
With 12 or more parties and some independents on the lists in different regions of the UK the ballot papers are large.
Staff in polling stations traditionally fold the ballot papers before handing them to voters to make it easier for the completed papers to be folded and posted into ballot boxes.
A spokesman for the Electoral Commission told BBC News they were "aware of the issue - and the general issues that long ballot papers can cause".
The commission said it was putting out an alert to returning officers after talking to returning officers and the UK Independence Party, which raised the issue with them.
A spokesman, who said ballot papers were folded to help protect secrecy, stressed the importance of people reading the instructions on ballot papers carefully.
A spokesman for UKIP said the party had already made a formal complaint - he said they had received calls from places including Carlisle, York, Hampshire and Dartford.
One man from York told the BBC he had been "absolutely shocked" that he could not find the party he wanted to vote for on the ballot paper and had to ask officials where it was.
"They explained you have to unfold it again, right at the very bottom there was another very neat fold that you could not see, folded backwards," he said.