Fortum is set to file its €8.05bn ($9.46bn) bid for E.ON’s fossil fuel spinoff Uniper with Germany’s financial regulator today, the company has revealed.

Details of the offer won’t be released until it has been reviewed and approved in mid-November, a company spokeswoman was quoted as saying.

The move follows on from E.ON’s agreement with Fortum to tender its 46.7 per cent stake in Uniper, worth €3.8bn, early next year.

Given that E.ON owns over 30 per cent of Uniper, under German law Fortum would be obligated to make a full acquisition bid if a deal with E.ON was agreed. Fortum said in September that it would offer €22 per share, with a €8.1bn valuation for Uniper.

Uniper’s management views the takeover attempt as hostile and Klaus Schäfer, Uniper’s CEO, has said the “unsolicited takeover offer is clearly not in line with the strategy of Uniper”.

Pekka Lundmark, Fortum’s CEO, countered in September by saying that Uniper’s response was “certainly quite unusual and a bit confounding”.

Fortum has said it is not interested in a full takeover, adding in a statement that “as Uniper has stated its intention to remain independent”, it “would focus on being an active, supportive and reliable shareholder of Uniper” and a “constructive strategic partner”.

However, the firm said the two businesses are “highly complementary”, calling Uniper’s hydropower and nuclear power plants in Sweden “an excellent fit with Fortum” and drawing a parallel between the two firms’ gas-fired fleets in Russia.

Fortum also pointed to Uniper’s gas-fired, coal-fired and hydropower plants in continental Europe, saying they “have an essential role in providing security of supply and affordable energy as Europe transitions towards low emission energy”.

“In addition, Uniper has extensive commodities trading activities that are complementary to the power generation business,” Fortum said.

Meanwhile, waiting in the wings is RWE, which is rumoured to be mulling buying Uniper assets from Fortum after the Finnish firm’s planned takeover, rather than launching a counterbid.