Video report by ITV News Correspondent Julient Bremner

EU citizens have been struggling to vote in today's European elections.

Thousands of angry voters posted on Twitter using the #DeniedMyVote hashtag to report they had been turned away from polling stations, despite believing they had registered to take part.

The problem comes down to an additional declaration which must be made by citizens of European nations if they wish to vote overseas.

Issues with overseas postal ballots, used by British Citizens living abroad to vote in elections for MEPs to represent the UK, have also been reported.

Italian voter Valentina Ferrandes told ITV News she thought the problem was an administrative issue that could be fixed in the polling station.

"I just asked for a new form, as you would expect, and I was just told 'no, we just can't do it, sorry, there's nothing we can do'," she said.

"I pretty much lost my vote for this European Election," she added.

Why have people been unable to vote?

People have complained online about not being able to take part in the ballot. Credit: PA

Citizens of European countries must fill in a European Parliament voter registration form to ensure they have the right to cast their vote in the country of their choosing. This must be completed and returned 12 working days before the polls open.

As the elections are staggered across several days in different countries, the form is a bid to prevent people from casting multiple votes in multiple countries.

For example: a French citizen living in the UK could vote in Britain, but would also be eligible to vote in France - the purpose of this form is to ensure they cannot cast two votes.

It appears the short notice for issuing guidance about the requirement to fill in the form, due to the UK's late decision to take part in the elections, is the reason behind the problem.

Others said they had returned the required paperwork, but it wasn't processed in time for the elections.

European nationals need to fill in additional paperwork to be able to vote in elections. Credit: PA

What about Britons living abroad who want to vote for MEPs to represent the UK?

British Citizens living overseas have the option to elect MEPs using a postal ballot, which they must return before close of polls on election day.

But there are reports such documents have not arrived - or arrived too late to be returned.

One British voter, based in France, told ITV News her local council in the UK told her "some other people in France had not received" the paperwork and it would look into the issue.

She said she has had no response from election officials or councillors, despite repeated attempts to contact them.

How many people have been affected?

It's unclear how many voters have been affected - but it could run into the hundreds of thousands.

Immigration and human rights lawyer Agata Patyna, who says she has lived in the UK since 2005, said she was turned away from her polling station because of the error.

She tweeted she had confirmed with her local council she would be able to vote, but received a different response on the day.

Peter Baier tweeted he had recieved a polling card and had registered to vote but was still turned away from a polling station in Hampshire.

He blamed East Hampshire District Council for failing to send out the correct paperwork ahead of the polls opening.

Where does the blame for people not being able to vote sit?

The Electoral Commission told ITV News the short notice decision for the UK to take part in the elections is the source of the issue, blaming central Government and Parliament for not ushering in legal changes it had suggested.

In a statement, the organisation said: “This legal process could be made easier for citizens, and the Commission made the case for doing so following the last EU elections in 2014. However, improvements to the process are reliant on changes to electoral law, which can only be taken forward by Government and Parliament.

“The very short notice from the government of the UK’s participation in these elections impacted on the time available for awareness of this process amongst citizens, and for citizens to complete the process.

"EU citizens’ right to vote in the election in their home Member State remains unaffected by the change in the UK’s participation; in order to do so, they would need to be registered in that country in accordance with that country’s process and timetable.”

The Local Government Association, which represents the UK's councils, told ITV News: "Councils are hugely experienced at running elections and have worked tirelessly around-the-clock to get everything in place for these EU elections at short notice.

"The fact that these elections appear to mainly be going smoothly is testament to their incredible hard-work and expertise."