Polish Prime Minister Donald Tusk on Tuesday said that he would take legal as well as political means to investigate abuses of power under the previous government.
The former European Council president won parliamentary elections in Poland on October 15, ending the rule of the conservative Law and Justice party (PiS).
"People want to know whether we will hold those who lost on October 15 accountable for their actions," the PAP news agency quoted Tusk as saying.
"No one will forget all the evil that happened" under the previous government, the prime minister reportedly added.
Tusk cited media reports stating that the partly state-owned oil company Orlen had collaborated with an entrepreneur who was alleged to have had close ties to the pro-Iranian Lebanese militant group Hezbollah.
The national public prosecutor's office confirmed in a press conference on Tuesday that it was investigating the allegation, as well as two other cases in connection with Orlen relating to the sale of refinery shares and a controversial company merger.
Public prosecutor Dariusz Korneluk told journalists his office was taking the cases seriously as they had implications for Poland's security.
In his speech, Tusk pointed out that Polish security interests could not be separated from those of the entire continent, which he said have never been so dependent on Poland.
With a view to the European Parliament elections in June, he called for Poland and Europe to be protected from those who want to undermine the EU and NATO from both the outside and within.