Lawyers for Harry Shindler, 94, and his co-claimant, Belgium resident Jacquelyn MacLennan, say they are being penalised for exercising their right of free movement across the EU.
Britons who have lived abroad for more than 15 years are prohibited from voting in British general elections and as a result cannot participate in the referendum on June 23.
But lawyers from Leigh Day solicitors argue that the referendum franchise has already been extended beyond what is normal for elections, and should also include expats.
"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," said Richard Stein, the Leigh Day lawyer leading the case.
He has filed papers with the High Court in London seeking an urgent judicial review of the compatability of the EU Referendum Act with European Union law.
Shindler, who fought in Italy during World War II and retired to the country in 1982, lost a challenge to the 15-year general election rule at the European Court of Human Rights in 2013.
"It leaves us speechless to think anyone can stand up in parliament and deny another Brit the right to vote," Shindler said this week from his home in Porto d'Ascoli.
A government spokesman said the franchise issue had been "debated, considered and agreed" by parliament.
He noted that in the long-term -- but not in time for the referendum -- ministers remained committed to a promise to scrap the 15-year rule.
A petition to allow all British expatriates to vote in the referendum, launched in November, has attracted almost 15,000 signatures.
