Thousands of EU nationals living in the UK could inadvertently lose their right to take part in the upcoming European elections because of widespread confusion over how to register, campaigners have warned, amid reports some European citizens are yet to receive their poll cards.

Existing rules stipulate that EU citizens must both register to vote and sign a special “UC1” form stating that they will note vote anywhere else in the EU, by 7 May.

But confusion has arisen because the UC1 form must be resubmitted before every election – even if the European national intending to vote has already signed and submitted that form previously, for example before the 2014 poll.

Campaigners representing EU citizens in the UK are concerned a lack of awareness about the process could disenfranchise thousands of EU voters with the deadline fast approaching.

The Electoral Commission has launched a publicity campaign to make EU nationals aware of the form but admitted it was only running in select regions of the UK.

There was also criticism that the crucial form is not available in electronic format, meaning applicants must download, print, sign and post the document to their local authority – all before the early-May cut-off date.

The Independent was approached by one woman whose family of 11 had all received their poll cards for the 23 May ballot apart from the one relative – an EU citizen.

Maike Bohn, a co-founder of EU citizen’s group the3million, said she had heard evidence of other European citizens failing to receive notification they were eligible to take part, and warned confusion around the UC1 form was likely preventing some from registering in time.

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She told The Independent: “The government and the electoral commission have a duty of care towards EU citizens living in the UK.

“Not only do these 3.6 million people face the administrative burden to apply to stay in their homes, they are also facing unnecessary barriers to be able to participate in democratic processes in the UK.

“To prevent long-time EU residents in the UK from exercising their voting rights in European Elections is somewhat ironic given recent calls for migrants to integrate better and for the EU to become more democratic.

“There will be a sizeable number of EU citizens who will be disenfranchised in what could be their last national vote in the UK.”

There was a dramatic decline in the number of non-British EU citizens registered to vote for the European Parliament elections between 2009 and 2014 – the figure fell from 1,043,629 registered to vote in the European elections in 2009 to 327,883 five years later.

Fears persist that this number could drop further in the upcoming poll.

Meanwhile, the government is facing the possibility of a legal challenge over the length of time EU nationals have been given to register.

Confusion surrounding the date of the UK’s departure from the EU means the UC1 form was only sent to two million European nationals on the electoral register a few weeks before the deadline – instead of the usual four-month period.

Roger Casale, secretary general of the New Europeans, which represents EU citizens in the UK, said: “Because of the very tight timetable, we would urge the government to allow those EU citizens on the electoral roll, who miss the current 7 May deadline, to fill in the required UC1 declaration forms when they arrive at their local polling stations on 23 May.”

He said: “We hope that the government will rapidly see the wisdom of taking these actions – and that it will not be necessary for us to consider seeking judicial review to ensure that people are able to fully participate in the democratic process.

“But if the government does not take rapid remedial action, we will of course have to consider taking legal action.”

The Association of Electoral Administrators (AEA) – an organisation representing election officials in the UK – said it had attempted to “discuss key potential European parliament election registration and organisational challenges” with civil servants on several occasions but had failed to secure any changes because, with Brexit looming, officials believed the UK would no longer take part in European elections.

“They were unwilling to engage with us on the subject as, in their view, there was not going to be an election in the UK,” said Laura Lock, deputy chief executive of the AEA.

The Electoral Commission also gave assurances to a Commons committee in 2014 that there would be changes to the system but plans were scrapped after the Brexit vote in 2016.

A spokesperson told The Independent any changes to the law regarding how European citizens register to vote “are a matter for the UK government and Parliament".