Tusk told reporters on Friday that an upcoming parliamentary vote on Climate Minister Paulina Hennig-Kloska would be a test of coalition loyalty.
"If it turns out you're not with us, we'll say goodbye," he said, addressing Poland 2050, one of three parties that make up his coalition government.
The vote – on an opposition motion to remove Hennig-Kloska – is due by 30 April.
The motion was signed by around 100 MPs from the far-right Confederation party and the opposition Law and Justice (PiS) grouping, who accuse the minister of mismanaging a clean air subsidy scheme and failing to protect Poland from the costs of EU climate policy.
Hennig-Kloska was originally nominated by Poland 2050 but left the party in February, which has complicated the coalition's internal dynamics.
Poland 2050's leader Katarzyna Pełczyńska-Nałęcz rejected Tusk's tone sharply.
"First snide remarks at the cabinet meeting, now an ultimatum in the media," she wrote on X.
"The language of ultimatums is a breach of the agreement. A coalition built on coercion is a road to disaster."
Speaking earlier in the day, Pełczyńska-Nałęcz said her party would make its own decision on how to vote, depending partly on whether outstanding questions about the clean air programme, including around PLN 2 billion (EUR 470 million) in unaccounted funds, are resolved in the coming two weeks.
The smaller coalition partner, the Polish People’s Party (PSL), has signalled it will vote in line with agreed position.
(ał)
Source: PAP