The leaders of a newly-formed Italian centrist group said on Thursday their pact will offer an alternative to "populist" rightist and leftist parties at a national election scheduled next month.

The Sept. 25 ballot was called following the collapse of Prime Minister Mario Draghi's broad unity administration in July after some key parties snubbed a confidence vote. The former European Central Bank chief resigned, but agreed to stay on as acting premier.

The centrist pact came after the Azione party led by Carlo Calenda surprisingly decided to quit the centre-left alliance, citing the presence of parties who failed to support Draghi as one of the reasons.

"A serious and pragmatic alternative to the right-wing and left-wing populism that has devastated this country ... was born today," Calenda wrote on Twitter after signing off on the deal with the Italia Viva party of former Prime Minister Matteo Renzi.

Azione and Italia Viva are polling at around 4% together. According to a study on Tuesday they would get just around 23 seats in the two houses of parliament, with the likely effect of boosting the overall conservative majority.

Polls show that a conservative alliance led by Giorgia Meloni's Brothers of Italy - which also includes Matteo Salvini's League and Silvio Berlusconi's Forza Italia - is poised to win an outright majority.

Italy's election law favours parties that form broad alliances.

The centre-left Democratic Party (PD) has also formed an alliance with several small centrist and leftist allies, while the 5-Star Movement will almost certainly run alone.

"Italy needs us to avoid the populist nightmare and return to dreaming of good politics," Italia Viva leader Renzi wrote on Facebook, saying Calenda will be the frontrunner of the centrist campaign.