The process had been postponed due to legislative changes in Moscow. The deal still requires a second Russian merger clearance as well as one from the EU, both of which are expected during the summer, Fortum said.

Fortum has agreed to buy a stake of up to 50 percent in Uniper. Both companies have large operations in Russia and Moscow's approval is considered as the key step in getting the deal done.

(Reporting by Jussi Rosendahl; Editing by Daniel Dickson)