In a separate statement, Fortum said it had also received merger clearance from the Russian Federal Antimonopoly Service (FAS) to acquire up to 50 percent of Uniper, in line with a previous decision disclosed at the end of April.

Fortum said the FAS decision would prohibit it from making unjustified price increases on the wholesale electricity market, adding that would not have a significant impact on its operations.

The approvals remove the remaining hurdles for Fortum to close the 3.8 billion euro ($4.4 billion) transaction, which the Finnish group said would take place no later than June 27.

The European Commission gave the green light without setting conditions, confirming a Reuters story earlier this week.

"We can approve their proposed merger, in particular because of the high level of interconnectivity between different countries in the Nordic area and because there is significant spare generation capacity in Sweden," European Competition Commissioner Margrethe Vestager said.

Uniper has opposed the transaction, arguing the combination makes little sense given the energy group's heavy exposure to gas and coal-fired power plants while Fortum's focus is on clean technologies.

Last week, Uniper Chief Executive Klaus Schaefer said he would defend the energy group's independence, suggesting a full takeover would not be possible with him in the driving seat.

(Reporting by Foo Yun Chee and Christoph Steitz; Editing Robert-Jan Bartunek and Mark Potter)