German weekly magazine Der Spiegel had said that insiders suspected Uniper was pushing back at least 100 million euros (87.9 million pounds) worth of impairments on the assets.
Fortum, which is controlled by the Finnish government, last year agreed to buy a 46.65 percent stake in Uniper from the company's former parent E.ON for 3.8 billion euros.
"The coverage by Spiegel today is wholly without basis," a Uniper spokesman said. "The speculation about an alleged tampering of our results is irresponsible and refuted by the assessment of an independent auditor. Many details of the coverage are incorrect."
Uniper last month disclosed a 270 million euro impairment on its Datteln 4 hard-coal power plant, saying it would have to replace the station's damaged boiler walls and delaying its commissioning to summer 2020.
Datteln 4, which was originally scheduled to open in 2011, has been a major headache for Uniper, suffering several delays and swallowing 1.2 billion euros in investment so far.
(Reporting by Christoph Steitz; Editing by Maria Sheahan)