"Soon the CETA (Comprehensive Economic and Trade Agreement) will come to parliament for ratification and the majority will reject it," Di Maio told Italy's main farming union, the Coldiretti.
"Any Italian civil servants abroad who continue to defend lousy treaties like CETA will be removed," he added.
"Being here means, in my view, reclaiming a bit of healthy sovereignty," said Di Maio, who heads the populist, eurosceptic Five Star Movement. He is joint deputy prime minister along with the Matteo Salvini, leader of the far-right League party.
"We must defend Italy and the Italian economy," he added.
The European Union and Canada formally signed the Comprehensive Economic and Trade Agreement (CETA) in October 2016, at a time when anti-globalisation sentiment was at fever pitch in Europe.
The accord eliminates 98 percent of tariffs between the EU and Canada. It needs to be ratified by all 28 members of the European Union in order to come into force.
Its opponents have long branded it as a danger to health, democracy and the rule of law. Farmers in Italy protested in 2017, demanding that the government scrap the pact. They wanted specialty products like Parmesan cheese to be labelled "Made inItaly".
CETA's supporters see the pact as an extension of the global trade system that faces a threat from protectionist US President Donald Trump.
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