Japan's main opposition party, the Centrist Reform Alliance (CRA), has decided to disband, following the failure of talks on a full merger.

Lawmakers formerly belonging to the Komeito party are set to return to it, while those from the Constitutional Democratic Party of Japan (CDPJ) plan to form a new party, the alliance's head Junya Ogawa told reporters on Wednesday in parliament after internal discussions.

Ogawa said the departing groups will seek to form a new parliamentary caucus under a centrist banner for an extraordinary Diet session this fall, in a bid to maintain their position as the largest opposition force in the lower house.

The decision came less than eight months after the CRA was founded in January as a counter to the ruling coalition ahead of the House of Representatives election in February, in which the new party suffered a heavy defeat.

A full merger involving the two founding parties' House of Councillors lawmakers and local organizations had been planned, but ultimately abandoned in August as the election loss weighed on talks.