Critics have said the acquisition of Polska Press from a German media group is part of a drive by Poland's nationalist Law and Justice (PiS) government to increase control over the media and curb free speech.
PiS has said that foreign media companies have too much influence in Poland and distort public debate.
"In the media all the time - not all of them fortunately, because some pluralism in the media has been restored, although unfortunately it is not yet full - we hear a story about defeat, about the fact that everything is wrong," PiS leader Jaroslaw Kaczynski told the party's convention on Saturday.
Poland's antimonopoly office, UOKiK, approved the takeover of Polska Press in February 2021, but the Human Rights Ombudsman, in appealing against the decision, argued the regulator had failed to assess the influence of the deal on media freedom.
"In our opinion, as a result of the acquisition of Polska Press by PKN Orlen, there is an unacceptable restriction of press freedom," Miroslaw Wroblewski an official at the ombudsman's office told Reuters. He declined to give specific examples ahead of the court's sitting, which is scheduled for June 7.
Since PKN Orlen took over Polska Press, most of its editors have been changed and many other journalists have also decided to leave.
"Such sweeping changes cannot be attributed to a series of mutually independent individual decisions, but are a pattern," Pavol Szalai, head of the European Union and Balkans desk for Reporters Without Borders, said.
PKN Orlen says the ombudsman's appeal is unfounded and denies any attempts at influencing Polska Press newspapers.
"From the very beginning, PKN Orlen has treated the purchase of Polska Press as an investment in business development and does not affect the editorial line implemented by the group's titles," the company told Reuters in an e-mailed statement.
"No editor-in-chief was dismissed, the contracts were terminated by mutual consent," it added.
Since PiS came into power in 2015, Poland has dropped to 66th place from 18th on the World Media Freedom Index.
(Reporting by Anna Koper; Editing by Bradley Perrett)