EU citizens are being denied the right to vote in European elections by confused polling station staff and incomprehensible bureaucratic requirements, campaigners have said.
And Brits living abroad in Europe are likely to face similar issues unless a radical overhaul takes place, politicians are being warned.
The Electoral Commission said it had received dozens of calls on the issue, and that it would be investigating concerns.
Local residents cast their votes at a polling station in the East Hull Boxing Club in Kingston-Upon-Hull
A Twitter account, @EUCitizensAbroad has retweeted numerous people saying they were barred from voting, and more than 500 people have signed a petition calling for an overhaul of voting rules.
"European citizens will be caught up in the massive confusion surrounding voting rights for the European election, resulting in potentially significant disenfranchisement and discouragement of participation in Europe’s largest democratic manoeuvre," said European Citizens Abroad's director of policy Natasha Marie Levanti.
"Today we have been hearing from hundreds of UK citizens or EU citizens in the UK who either themselves were denied their right to vote or watched a spouse, friend OR neighbour get denied their right to vote.
She called the system "a bureaucratic maze", and added: "What we are noticing is that it seems that more EU citizens who have been residing in the UK for long periods were likely to be caught up in the confusion yesterday."
The Electoral Commission said officials had been reminded they needed to send out additional forms to non-British EU citizens, asking them to declare they would not vote in their home country. "People are telling us that they didn't receive them and that's something we'll reflect in our post-election report. That's a concern we'll follow up on," a spokeswoman told HuffPost UK.
Joanna Papageorgiou, a Greek citizen who has lived in the UK since 1995, has written to her local Bristol MP Stephen Williams about being barred from voting for the second time since moving here. "In the previous local elections, I was told I could not vote because I was European even though I had my polling card with me. I wrote to the electoral commission and they told me that the information I had received was wrong," she explained.
This time, Papageorgiou who is a senior analyst at UCAS, said she tried to make sure everything was done by the book. "I registered to be on the electoral roll last year, as soon as I moved to my new flat. The same as I do every time I have the option. I have always registered," she told HuffPost.
"I received a letter two months ago asking me to declare that I was not going to vote in any other regional election and only in the UK one. I filled out the form and signed it."
She received no polling card but turned up to her local polling station regardless. "I was told I could not vote because I was registered to vote in a different country," she said.
Dr Rita Jorge, a 35-year-old Portuguese biologist who has lived in the UK for the past eight years, was informed she could only vote in local elections. Jorge was told she should have contacted the electoral register stating she was an EU citizen and applied for a second polling card.
"This information was clearly stated in the form I returned to them. They knew I was a EU citizen."
Jorge has complained to the Electoral Commission but said she "did not get a clear answer exactly what should have happened and how I could have made it happen".
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"I had a distinct feeling that they were deeply embarrassed by this," she told HuffPost UK, saying the Commission admitted there was no mention of a second polling card for EU citizens on its website or Jorge's local council website.
"Many voters like me were never informed that they wouldn’t qualify to vote unless they returned additional information, and it now transpires that for some that information was 'lost in the post'."
"My grandparents and my parent's generation fought for the right to vote as my country was ruled by a dictatorship for almost 40 years, until 1974," she added. "I take voting seriously. I was forced to migrate in 2006 due to the harsh economical climate lived in Portugal. This was my chance to have my voice heard, and to affect the policies which will have a deep impact on whether or not I will ever be able to return back home.
"I am deeply saddened about this, and willing to take this to the European parliament as a formal complaint."
Josefine Frank, a German trainee curator in Birmingham, was told she could only vote in the local elections for a similar reason. When she complained, the council office said she hadn't replied to their letter asking whether I wanted to vote here or in Germany. "I never received such a letter from Birmingham City Council," she told HuffPost UK. I sent a voter registration form, copies of my passport and proof of address.
"Being able to vote is very important to me so I feel disappointed and very angry," she added. "I read a daily newspaper, watch the news, use social media and checked my Council's website to get information on candidates I never saw anyone anywhere addressing this potential issue for non-UK EU citizens.
"No one at the polling station checked my passport or wanted any other form of identification, opening the floodgates to people to vote multiple times. However, when it comes to European elections the possibility for non-UK EU citizens to potentially vote twice needs to be eliminated by denying them their right to vote at all."
Citizens of the European Union can vote in European and local elections in the UK but not General Elections. But there is no one system for voting across EU nations, and elections are controlled by 28 different national authorities, with more than 100 different possible scenarios for the process of registering.
In January this year, Britain was accused by Viviane Reding, the EU’s Justice Commissioner, of “punishing” its own expats by stripping them of the right to vote if they had not been on the electoral roll for the past 15 years.
Reding said that those who work or retire abroad are treated as “second class citizens”.
“Depriving citizens of their right to vote once they move to another EU country is effectively tantamount to punishing citizens for having exercised their right to free movement. Such practices risk making them second-class citizens," she said.
Chris Grayling, the Justice Secretary, said the criticism was more European interference in British affairs.