The move comes amid the biggest split in PiS in more than two decades.
Morawiecki founded the Development Plus (Rozwój Plus) association in April as a more moderate faction within the party, but PiS leader Jarosław Kaczyński demanded its dissolution and ordered members to sign a pledge disavowing "external" groupings.
Around 40 of Morawiecki's allies refused, and PiS referred them to its disciplinary officer rather than expelling them outright.
Speaking on the YouTube programme "Rymanowski Live," Morawiecki was responding to comments from PiS disciplinary spokesman Karol Karski.
Earlier on Monday, Karski said the party membership of 40 lawmakers was now "dead" in practice, even though they remained formal members.
The group has already formed a separate parliamentary club.
According to Morawiecki, the remarks confirmed that PiS leadership had "wanted to push us out from the very beginning."
He added: "Now that our membership is dead – meaning we've been completely removed from Law and Justice – I can say we will indeed found this party by 15 October at the latest."
The date coincides with a party convention already announced by Development Plus, timed to mark the third anniversary of Poland's 2023 parliamentary election.
(ał)
Source: tvp.info