Fortum last year agreed to buy a 46.65 percent stake in energy group Uniper from E.ON for 3.8 billion euros (3.3 billion pounds), despite opposition from Uniper's management which has argued the two companies are not a good fit.

Last month, Fortum won approval from a government commission in Russia to buy up to 50 percent in Uniper, marking a key step in getting the deal done, but Fortum also said Uniper's management had actively worked against the deal there.

"We've done nothing wrong," Uniper CEO Klaus Schaefer told analysts during a call to discuss first-quarter results, adding the company's behaviour had been open and transparent with regard to both Fortum and Russian officials.

"In view of the situation after the decision in Russia, which is tolerable for both sides, I'm not quite seeing why Fortum appears to be somewhat annoyed," Schaefer added.

A Fortum spokeswoman said the company stood by its previous accusations, but declined to comment further. It expects to get final regulatory clearances for the acquisition by the middle of the year.

Uniper shareholder Cornwall Luxembourg S.a.r.l., which Schaefer says is backed by activist investor Elliott Management, will propose appointing a special auditor at Uniper's annual general meeting on June 6 to identify possible breaches of duty and violations of the law by members of the management board.

Fortum said it had nothing to do with that motion.

Uniper has advertised in Finnish newspapers, arguing against the transaction. It is being advised by Morgan Stanley and Rothschild on the deal, sources have told Reuters.

(Reporting by Christoph Steitz in Frankfurt and Jussi Rosendahl in Helsinki; Editing by Maria Sheahan and Mark Potter)