"We believe that fundamental principles of the law covering Italians living abroad have been violated," Alfiero Grandi of the 'No' committee said.
"We'd be very upset if the 'Yes' win by a small margin [due to those votes]; there are grounds for an appeal."
Italians will go to the polls on December 4 to vote on the government's proposed overhaul of Italy's political machinery.
Premier Renzi is widely expected to step down if he loses.
"We'll campaign on the merits of the referendum - they won't, because if you go into the merits it's game over; they're trying to turn it into a row, so keep calm and carry on," he said.
The young reformist premier added that the wide assortment of 'No' campaigners "are out of ideas" and are "competing against each other", while a 'Yes' vote "will help us change Europe".
Mr Renzi also dismissed opinion polls showing a clear lead for the 'No' campaign, and warned that nixing his government's reform would consign the country to more decades of political gridlock.
With ANSA
