ROME (REUTERS) - Italy's centrist Azione party will leave a centre-left election alliance it formed with the Democratic Party (PD) earlier this week ahead of a Sept 25 ballot, Azione's party leader said on Sunday (Aug 7).

Azione's decision to withdraw comes a day after the Italian Green leftist federation and centrist party Impegno Civico, agreed to join the PD-led alliance, a move that was seen as strengthening the centre-left which is already lagging behind conservative rivals.

Polls show that a conservative alliance is set to win next month's election, with far-right the Brothers of Italy set to be the largest single party. Italy's election law favours parties that form broad alliances.

Azione leader Carlo Calenda said on Sunday that he had told the PD's leaders Azione would leave the accord, citing the presence of parties who voted against the Draghi government as one of the reasons.

The September vote was called after the unity government led by Prime Minister Mario Draghi collapsed last month. Mr Draghi resigned but has stayed on as acting prime minister.

"This has been the most painful decision of my life," Mr Calenda told state-owned television channel Rai Tre.

Azione had agreed to team up with the PD, the largest party on the centre-left, in an effort to make up ground on the conservatives, pledging to stick to Mr Draghi's foreign policy of supporting Ukraine and to meet targets required to access billions of euros in funding from the European Union.