Currently, expats who have not been resident in the UK for over 15 years are ineligible to vote in general elections. Despite hope that the restriction would before the referendum, it remains in place, much to the anger of long-term expats.
This group is estimated to number 2.2 million - almost half of the five million Britons currently living abroad.
Lawyers will lodge the claim at Britain's High Court on Wednesday, acting on behalf of a group of expats, which includes pensioners' rights campaigner 94-year-old war veteran Harry Shindler, who lives in Italy and has previously petitioned the European Court of Human Rights (ECHR) about the law, and Brian Cave, who lives in France.
Shindler, who lives in the Marche hamlet of Porto d'Ascoli, has for more than twelve years fought to reclaim his right to vote.
“This really is the last gasp, there will be nowhere to go after this,” he told The Local.
“It's very important. To say we can't vote on an issue that concerns us is wrong.”
Still, Shindler said he was "very confident" about the outcome of the challenge.
One of the reasons being is the fact that members appointed to the House of Lords, the upper house of the UK parliament, who alongside British people overseas for more than 15 years are not allowed to vote in a general election, will be able to cast their vote in the upcoming referendum.
“The British government is telling us they're applying the same rule used in the general elections – but this is not correct. The rules are being bent for members of the House of Lords.”
Giving long-term expats the right to vote is also “common sense”, he added.
“These are the people who are more concerned with the outcome on the referendum than anyone else."
Lawyer Richard Stein from Leigh day said the firm's clients "are being penalized for exercising their EU free movement rights".
"The people it arbitrarily excludes are those UK citizens who are among those most likely to be affected by the decision taken by voters in this referendum," he added in a statement.
“Not to allow them to vote on the decision whether the UK remains part of the EU is unlawful and we have asked the court to deal with the issues urgently so that the act can be amended before the June date, to include all UK citizens residing in the EU for however long.”
Charlotte Oliver, an English lawyer based in Rome who runs her own practice in the Italian capital and supports the legal action being taken, said:
"I am one of millions of people who make up the ever-increasing international British community, having lived in Italy since 2001 for family reasons. However, I still consider the UK my home, and the place where I will retire one day.
"As the Conservative Party made clear in their manifesto, there should be a vote for life for British citizens. The cut off point of 15 years is arbitrary and arcane given travel and communications have improved so much since it was imposed in 1982.
"It does not prove that a person has severed all ties with their country of origin. I cannot vote in Italian elections either as I am not an Italian national, so I am disenfranchised.”
The case, if successful, could have a huge impact on preparations for Britain's EU referendum on June 23rd.
At the last UK general election in 2015, 106,000 Brits who had been resident abroad for less than 15 years registered to vote.
While expatriates who have lived abroad long-term may not be significantly affected by the results of general or local UK elections, the Brexit referendum would have a huge impact on their lives.
David Cameron's Conservative Party also pledged to "introduce votes for life, scrapping the rule that bars British citizens who have lived abroad for more than 15 years from voting". However, this change will not come into force before the referendum.
If successful, the referendum scheduled for late June may have to be delayed while the extra names are added to the electoral register.
If you are a British expat and have been living in Italy for less than 15 years, you can vote in the EU referendum on June 23rd. To register CLICK HERE.