The offer was among 21 compromise points Morawiecki presented to PiS leader Jarosław Kaczyński on Thursday, after party leadership set a midnight deadline for MPs to declare they belong to no outside political associations, under threat of expulsion.
The demand is aimed chiefly at members of Rozwój Plus (Development Plus), an association founded by Morawiecki.
Thursday's talks between the two politicians ended without resolution.
Kaczyński was due to make a statement at noon on Friday.
Speaking to private broadcaster wPolsce24, Morawiecki said the idea of a "loyalty declaration" carried very negative connotations for him.
He added his proposals also included pledges not to build a rival political structure and to coordinate more closely with the party leadership – concessions he said Kaczyński had called "far-reaching."
According to Morawiecki, some within PiS were focused mainly on "intrigue", and expressed surprise that MP Jacek Sasin had joined a meeting he had expected to be one-on-one with Kaczyński.
PiS MEP Piotr Müller, a fellow Rozwój Plus member, called Sasin the "gravedigger" of a possible deal and, he said, the "godfather" of a future election win for Prime Minister Donald Tusk.
Müller added that PiS was increasingly influenced by former members of Zbigniew Ziobro's Suwerenna Polska (Sovereign Poland) party.
Morawiecki said he remained ready for further talks and ruled out political cooperation with any grouping in Poland's governing coalition.
(ał)
Source: PAP