During a meeting in Paris, the French president and former Italian prime minister agreed to present a “common front” against anti-EU politicians.
They spoke about how to counter the rhetoric of Matteo Salvini, leader of Italy‘s Lega Nord, and far-right French MP Marine Le Pen.
A source last night said: “The relationship between Macron and Renzi showed itself to be strong and consolidated.
“They agreed on the need to combat populism, Le Penism and its allies, including Salvini in Italy.”
But Mr Salvini hit back, saying Mr Macron and Mr Renzi should be focusing on more important issues facing them.
He said: “Instead of worrying about Salvini and Le Pen, it would be more useful for Renzi and Macron to worry about Islamic terrorism, high unemployment levels, and Europe.
“Socialists and democrats? No, just ridiculous.”
Mr Renzi, who quit as Italian PM last year, hailed the rookie French leader following their hour-long talk at the Elysee Palace.
He said: “Macron is the point of reference for rebirth of the European Union.”
The pair are also understood to have expressed “concern over the uncertainty of the situation in Germany“.
German Chancellor Angela Merkel has yet to form a coalition after failing to win an outright majority in September’s election.
Mr Macron was elected France’s youngest ever president earlier this year after vowing to reform the crisis-hit EU.
He comprehensively beat Ms Le Pen, the then-leader of the National Front, picking up 66 per of the votes in July’s poll.
But the 39-year-old’s popularity has plunged after a series of unpopular policy announcements, including proposed budget cuts.