By Federico Maccioni and Angelo Amante

ROME (Reuters) -The leader of Italy’s centrist Azione party said on Sunday it would leave a centre-left election alliance it formed with the Democratic Party (PD) last week, dealing a blow to the coalition’s odds ahead of a Sept. 25 ballot.

The Green leftist federation and centrist party Impegno Civico had only the day before agreed to join the PD-led bloc, a move that was seen as strengthening a centre-left already lagging behind conservative rivals.

Polls show that a conservative alliance is poised to win next month’s election, with the far-right 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 he had told PD leaders his party would leave the accord, citing the presence of parties who voted against former Prime Minister Mario Draghi’s government as one of the reasons.

The September vote was called following the collapse of the Draghi’s unity government last month, after three main partners snubbed a confidence vote he had called to try to end divisions. Draghi resigned but has stayed on as acting premier.

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

PD leader Enrico Letta tweeted: “I listened to Carlo Calenda. From all the things he said, it looks to me as if the only possible ally for Calenda is Calenda (himself).”

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. He pledged to stick to Draghi’s foreign policy of supporting Ukraine and to meet targets required to access billions of euros in funding from the European Union.

The centrist party and its +Europa ally are polling at around 5-7% in surveys. Earlier on Sunday, +Europa expressed its strong approval of the pact with the PD and it remained unclear what the small group would do after Calenda’s move.

(Reporting by Federico Maccioni and Angelo Amante, editing by Raissa Kasolowsky and Mark Heinrich)