Harry Shindler, 94, had fought to change a law that barred British expats who have lived abroad for more than 15 years from voting in the crucial referendum.
But his appeal was rejected by the UK's highest court in late April, effectively blocking some two million Britons living in the EU from having an input.
“I've spent the morning weeping,” he told The Local.
“I think it's a disaster for Great Britain. It will have a very bad effect on Britons in the EU and it could have been avoided. David Cameron will rue the day he refused to give us the vote – an economist advised him to give us the vote – those who voted ‘leave' will rue the day too.”
British Prime Minister David Cameron, who backed ‘Remain', stepped down on Friday morning.
Shindler, a veteran of the Battle of Anzio and the liberation of Rome in the Second World War, said the more worrying thing about the outcome of the referendum was that those who voted ‘Leave' made their decision based on “minor issues”, dismissing the 70 years of peace the country and continent have enjoyed thanks to the EU.
A staggering 59 percent of those who voted ‘Leave' were over the age of 65, compared to over 70 percent of those aged between 18 and 24 who voted 'Remain'.
“All those who lived through bombings in London, Plymouth, and everywhere else...would have remembered how bad it was and should not want that to ever happen again,” Shindler said.
“I would have thought that 70 years of peace would have been sufficient for people to say ‘we're not going to change' anything. The matter of peace is far more important than anything else.”
Shindler, who has lived in Italy since 1982, was awarded an MBE in 2014 for his work in helping to find the graves of British soldiers killed or listed as missing during the war.
In the High Court bid, Shindler and other British expats had argued that rules governing the UK general elections, the basis for the referendum vote, were not equally applied.
For example, 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, were able to cast their vote in Thursday's referendum.
But Shindler has vowed to continue his fight to ensure that a tabled UK bill that would give long-term expats a ‘vote for life' is passed.
“I now need to make certain that the ‘vote for life' bill is not lost,” he said.
“If, in fact, I find that the bill will be pushed aside then I will have no alternative but to take it to the UN human rights committee. I hope it doesn't come to that, but that's what I will do.”