Renzi says two-thirds majority of Italians living abroad would shift balance in favour of yes camp before Sunday’s vote

Millions of Italians who live abroad will have their last chance to cast their vote on Thursday in a critical referendum on constitutional reform brought by the prime minister, Matteo Renzi.

Renzi, who has promised to resign if he loses, faces a five-point deficit in the polls heading into the final week of campaigning for the 4 December referendum, which centres on sweeping changes to Italy’s parliamentary system. But Renzi told a Belgian newspaper that early voting by Italian emigrants could help clinch a victory.

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“If the yes [camp] manages to win the approval of two-thirds of Italians abroad, then we can do it,” he told Le Soir, in remarks that were picked up by Italy’s daily newspaper, La Repubblica.

There are about 4 million eligible Italian voters abroad, of which about 1.1 million voted in 2013’s general election. Renzi calculated that if 1.5 million voted in the referendum, and the yes camp won a million of those votes, “the balance would shift”.

But there are potential problems with Renzi’s maths. Primarily, though Renzi’s position is on the line, the referendum is not a national election and could attract less interest among Italians living all over the world.

A diplomatic source at the Italian embassy in London – the city with the second-highest number of registered Italian expatriates – said half of all eligible voters in the UK had submitted their ballots by Monday, and more were expected by the end of Thursday.

“In general, the [expatriate] community in London has been active [on the referendum]. Over 60% have a higher education degree and they’re interested in politics,” the official said.

He added that the referendum had received a lot of media attention in the UK, where resident Italians were more politically engaged with their homeland, in comparison to Buenos Aires, the city with the most registered Italian emigrants.

Facebook Twitter Pinterest Beppe Grillo, leader of the Five Star Movement, is campaigning hard for a no vote. Photograph: Gregorio Borgia/AP

Italians living abroad are considered to lean more towards the yes camp in the referendum, but each side campaigned outside of Italy to target those voters. Both Maria Elena Boschi, Renzi’s reform minister, and Luigi Di Maio, a star of the anti-establishment Five Star Movement, which is leading the opposition against the referendum, appeared in rallies in London to help get out the vote.

Italian emigrants have been decisive in two elections since winning the right to vote by mail in 2001, according to an analysis by Lorenzo Piccoli, a researcher at the European University Institute in Florence: the tight election of 2006, which ended in a victory for Romano Prodi’s centre-left government; and the 2013 election in which the centre-left secured a majority.

Referendum ballots from emigrants are sent to a processing centre in Castelnuovo di Porto and will be counted on Sunday, the day the rest of the country goes to the polls.

The Renzi government was criticised last month for sending emigrants a signed letter from the prime minister urging them to vote yes, along with a two-page brochure in whichRenzi was pictured with Barack Obama and Angela Merkel.