The High Court has reserved judgment on whether British citizens living in other European countries are legally entitled to vote in the European Union referendum.
Two judges sitting in London heard urgent legal challenges brought by 94-year-old Second World War veteran Harry Shindler, who lives in Italy, and lawyer and Belgian resident Jacquelyn MacLennan.
They are seeking a declaration that Section 2 of the EU Referendum Act 2015, which denies them a vote because they have lived abroad for more than 15 years, is incompatible with their EU law rights.
After a day-long hearing, Lord Justice Lloyd Jones, sitting with Mr Justice Blake, said the court would take time to consider its decision.
The judge said the court "appreciates the importance and urgency of the matter" and would produce its judgment as soon as possible.
David Eadie QC, appearing for the Government, warned that a win for Mr Shindler and Ms MacLennan could lead to it being impossible to hold the referendum as planned on June 23.
But Aidan O'Neill QC, appearing for both applicants, said the Government claim was exaggerated, adding: "There is plenty of time for the necessary changes to be done."
Mr O'Neill said both his clients were "not expatriates" but "Britons in Europe" exercising their right to free movement within the EU.
"Without British membership of the EU these two British citizens will no longer be EU citizens and therefore no longer have the rights which flow from EU citizenship," he said.
They would become "resident aliens" and their right to live and work in their host countries would be placed in jeopardy.
He said the Government had no idea how many people were being disenfranchised.
It was estimated there were between one million and two million British citizens currently living in other EU countries.
Mr O'Neill said there were no figures for how many were caught by the 15-year rule - "but we are talking substantial numbers".
Asking the court to rule the legal challenge "unarguable", Mr Eadie QC said the 2015 referendum legislation was not open to challenge on EU grounds.
In any event the 15-year rule did not, as a matter of fact, interfere with EU free movement rights.
Mr Eadie said the impact of a "leave" vote on those caught by the rule "would be subject to negotiation with other member states and cannot be predicted at this stage".
Mr Shindler, who retired to Italy in 1982, has previously said the legal challenge was the ''last stand'' for individuals fearing for their way of life if the UK severs ties with Brussels.
The veteran, who is not at London's High Court for the hearing, said he would vote Remain if he had the chance as many expats were concerned about the future if the UK voted to leave.
''It would have very serious repercussions for all expats and their families here,'' he told the Press Association recently.
Mr Shindler, who fought in Italy during the Second World War and was part of the Anzio landings in 1944, said younger expats could be forced to apply for work permits to remain in their adopted countries.
He was sceptical about assurances from the Leave camp that there would be little change in the status of Britons living in the EU following Brexit.
Ms MacLennan, who is based in Brussels, said before going into court: "I am very disturbed that my fundamental democratic rights could be removed by an ill-conceived piece of legislation like this.
"The arbitrary removal of those rights because I have lived outside the UK for 15 years, though maintaining close links with the UK, seems to me to be something right-minded people should consider to be a problem."
