The date of the European Union referendum will be put in doubt if British citizens living in Europe win a legal battle for the right to join the vote, Government lawyers have told the High Court.
It would be impossible for the necessary steps to be taken without calling into question the date, they argued.
Two judges, sitting in London, are hearing an urgent legal challenge to rules which deny the vote to Britons who have been living in other European countries for more than 15 years.
The application for judicial review has been brought by 94-year-old Second World War veteran Harry Shindler, who lives in Italy, and lawyer and Belgian resident Jacquelyn MacLennan.
Both claim that under the EU Referendum Act 2015 they are unlawfully being denied the right to vote on the UK's continued membership of the EU, despite the impact it could have on their lives.
Aidan O`Neill QC, appearing for both applicants, said they were "not expatriates" but "Britons in Europe" exercising their right to free movement within the EU.
The QC told Lord Justice Lloyd Jones and Mr Justice Blake: "Precisely because their British citizenship brings with it their European Union citizenship they are absolutely dependent on that.
"Without British membership of the EU then these two British citizens will no longer be EU citizens and therefore no longer have the rights which flow from EU citizenship.
"But they have been deprived of the right to participate in this referendum because they have exercised their free movement rights for too long."
Mr O'Neill is asking the judges to rule there is no good reason for denying his clients the vote, and the Government cannot legally justify the current interference with their EU law free movement and citizenship rights.
He said the Government had no idea how many people were being disenfranchised. Numbers of British citizens living in Europe were estimated at between one million and two million, but there were no figures for those who had been there for more than 15 years and denied the vote - "but we are talking substantial numbers".
Mr O'Neill also argued there was no merit in the Government claim that the legal challenge had been brought too late and that it was unrealistic to expect the franchise to be extended while the June 23 date of the referendum was maintained.
The obstacles to abolishing the 15-year rule were being "exaggerated", said the QC.
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.
''I came here in 1982 when you had to have a permit from the police to stay here. All that would come back. We would be immigrants here.''
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."
Richard Stein, of solicitors Leigh Day, who is representing Ms MacLennan and Mr Shindler, said if their challenge was successful it would require the Government to rush through amending legislation to change the franchise for the referendum.
Mr Stein said: ''We believe that the Government has the time now to amend the franchise and empower the many expats who have lived outside the UK for over 15 years who want to vote on decisions which will have a very real impact on their lives.
''This legal action should not delay the referendum, the Government should instead stand by its promises and give a 'vote for life' to British citizens.''
In court James Eadie QC argued on behalf of the Government that 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".
Many of those affected would have family connections and other bases entitling them to continue to live and work in other EU countries regardless of their UK citizenship, and so regardless of the outcome of the referendum.
Mr Eadie added it was correct that the Government was committed to repealing the 15-year rule and had described it as an "arbitrary" cut-off.
But that did not undermine the arguments he was putting to the court. The cut-off could be described as "politically arbitrary" without that amounting to any form of legal concession.
A Cabinet Office spokesman later stated: "The franchise for the EU referendum was debated, considered and agreed by both Houses of Parliament within the last 12 months.
"Scrapping the 15-year rule for overseas electors is a manifesto pledge which the Government remains committed to delivering.
"This is a permanent change to the election franchise and is not connected to the referendum."