The president of the European Council said Brussels’ trust in Warsaw was now “dead” due to an escalating crisis sparked by a crackdown on democracy and a judicial overhaul in Poland.
And he warned the ruling Law and Justice (PiS) party was aligning the country with Russia, to behave like “twins”.
Mr Tusk warned PiS could pull his home nation from the union unless there were sufficient “benefits” to membership.
He and said the party already thought of an exit as a “goal”.
Mr Tusk, the former Prime Minister of Poland, said: “I have no doubt that for PiS one of the goals is to ‘free’ Polish politics from the burden of the EU.
“I can easily imagine a situation when Poland finds itself among the net payers, then the government in Poland would feel it is time to ask Poles if they want Poland to continue being in the EU.
“For PiS, the benefits of being in the EU relate only to the balance of payments.”
He said he hoped Poland would remain in the EU – but would not take such a conclusion for granted.”
He added: “In Brussels there is still enormous hope, I won’t say trust because that’s already dead, that Poland will stay in the Union.”
Mr Tusk also accused Poland of working closely with Russia.
He said: “Almost everything that the ruling party in Poland proposes in key political issues matches the publicly formulated and described interests and expectations of Moscow.
“I could name a lot of cases in which PiS and Moscow have similar views and goals, like twins.”
Poland has regularly clashed with the EU in the last year.
Last month the EU even took steps towards stripping Poland of its voting rights in reaction to a concerning overhaul of the country’s judicial system.
Mr Tusk said it was a “sad day” and Commission Vice President Frans Timmermans said the decision was made with a “heavy heart”.