The bill from Prime Minister Donald Tusk's centrist government, first presented to parliament in December last year, aimed to give unmarried couples - including same-sex couples - expanded rights.
Poland is, along with Bulgaria, Romania and Slovakia, one of the last countries in Europe that has not legalized either same-sex marriage or civil partnerships.
"As the guardian of the constitution, I cannot accept a solution that would lead to the loss of the special status of marriage, a marriage defined as a union between a woman and a man," Nawrocki said in a statement.
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