Attempts by Italy's centre-left parties to form an alliance for September elections were in disarray Monday, boosting the chances of the hard right taking power in the eurozone's third-largest economy.

Just days after agreeing a pact with the centre-left Democratic Party, the smaller Azione party withdrew, saying it could not work with left-wingers brought in to boost the alliance.

"You cannot explain (to voters) that to defend the constitution you make a pact with people you know you will never govern with," Azione leader Carlo Calenda, a moderate, told Corriere della Sera daily.

The Democratic Party, led by former premier Enrico Letta, is running neck-and-neck in opinion polls with Giorgia Meloni's post-fascist Brothers of Italy ahead of general elections on September 25.

But Italy's political system favours coalitions, and while Meloni has a strong alliance with Matteo Salvini's anti-immigration League and Silvio Berlusconi's Forza Italia, Letta is struggling to bring together the disparate progressive parties.

Letta accused Calenda of being unwilling to compromise with anyone, and also railed at former premier Matteo Renzi, leader of the Italia Viva party, for refusing to join his alliance.

"It's a way to help Meloni and Salvini," Letta told La Stampa newspaper, while adding that talks were now over, and he would focus on the campaign.

Letta on Saturday announced an agreement with left-wing Sinistra Italiana and greens Europa Verde, as well as Foreign Minister Luigi Di Maio's new party Civil Commitment.

Di Maio recently defected from the Five Star Movement, which triumphed in the 2018 elections but has since suffered a wave of defections and a collapse in public support.

The elections were triggered by the resignation of Prime Minister Mario Draghi last month following the implosion of his national unity coalition.

The Five Star Movement, Salvini's League and Berlusconi's Forza Italia had all withdrawn their support.

The elections come at a sensitive time for Italy as it faces soaring prices, particularly in energy, and implements deep structural reforms to secure billions of euros in European Union post-pandemic funds.