Two expats are challenging a decision to bar British citizens who have lived elsewhere in Europe for more than 15 years from voting in the EU referendum.
Harry Shindler, a 94-year old World War Two veteran and Jacquelyn MacLennan say June's vote will directly affect them.
They argue that they have taken advantage of EU rules allowing freedom of movement and could lose these rights if Britain votes to leave the union.
The government said the franchise was agreed by both Houses of Parliament.
The campaigners say up to two million expats are being denied the right to take part in the referendum.
Their lawyers will file papers at the High Court on Wednesday requesting a judicial review which, if successful, could force the government to pass emergency legislation before the EU vote on 23 June to change the franchise.
Under current rules, British nationals who have lived abroad for more than 15 years are unable to vote in UK elections.
The Conservatives have pledged to scrap the 15-year rule, but this has not yet been delivered.
Mr Shindler - who has lived in Italy since 1982 - and other campaigners argue the 15-year cut-off is arbitrary and that rules governing UK general elections, the basis for the referendum franchise, are not being applied evenly.
Law firm Leigh Day, which is representing Mr Shindler and Belgium resident Mrs MacLennan, says the 2015 EU Referendum Act authorising the vote extends the right to vote to peers, and Gibraltar residents who would not normally be able to take part in general elections, but not long-term expats.
"Our clients are being penalised for exercising their EU free movement rights," said Leigh Day's Richard Stein.
"The EU Referendum Act 2015 is said to be based on legislation for UK parliamentary general elections. But it gives a vote in the EU referendum to members of the House of Lords, as well as to Irish and Commonwealth citizens who are resident in Gibraltar.
"None of these are allowed to vote in UK general elections. 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.
"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."
Mr Shindler has long campaigned for British expats like himself to be able to vote in the UK, arguing he maintains strong ties with his country of birth but has not been able to cast a vote since 1997.
In 2013, he lost a legal challenge at the European Court of Human Rights, which ruled that the UK should have "room for manoeuvre" over eligibility for voting and its laws were proportionate.
A Cabinet Office spokeswoman said: "The franchise for the EU referendum was debated, considered and agreed by both Houses of Parliament and is set out in legislation.
"Scrapping the 15-year rule for overseas electors is a manifesto pledge which the government remains committed to delivering.
"It is not connected to the referendum."