* Elliott stake comes amid planned takeover by Fortum
* E.ON can sell its stake to Fortum for 22 euros/shr
* Uniper shares close up 3.9 pct after hitting record (Recasts, adds context on E.ON-Fortum deal, shares)
FRANKFURT, Dec 5 (Reuters) - Activist investor Elliott Management has taken a 5.32 percent stake in Uniper, putting pressure on Finland's Fortum to sweeten its 8.05 billion euro ($9.51 billion) takeover offer for the German energy company.
The bet marks Elliott's latest foray into Germany's M&A arena after the fund took a stake in drugmaker Stada and successfully extracted a higher offer for the group from private equity firms Bain Capital and Cinven.
Elliott's stake in Uniper comprises a voting equity stake of 2.39 percent, with a further 2.93 percent held through unspecified instruments, Uniper said on Tuesday.
The stake is worth 480 million euros based on Uniper's closing share price on Monday.
The move comes ahead of plans by E.ON, Uniper's former parent, to tender its remaining 46.65 percent stake in Uniper to Fortum for a fixed price of 22 euros per share by Jan. 11, 2018.
Fortum last month submitted a bid for the whole of Uniper to comply with German takeover rules. Uniper's management rejected the offer, saying it substantially undervalued the group and made no strategic sense.
Fortum, which has a strong focus on clean technologies as opposed to Uniper's exposure to coal- and gas-fired power plants, previously said that its primary aim was to acquire E.ON's stake in Uniper. It has ruled out an increased offer.
Fortum, 50.8-percent state-owned, was not immediately available for comment. Elliott declined to comment.
Uniper's shares closed 3.9 percent higher at 25.88 euros on Tuesday after earlier hitting 25.995 euros, their highest level since the group's listing in September 2016.
E.ON can still withdraw from its deal with Fortum but that would trigger a hefty 1.5 billion euro compensation payment, making it difficult for E.ON to pull out unless Uniper's shares rise above 30.80 euros.
E.ON, which spun off Uniper to focus on renewables, networks and power retail operations, confirmed on Tuesday it would make a final decision on the sale of the Uniper stake in January.
Elliott's strategy of pushing for higher offers in takeover situations has not always worked out. General Electric last year walked away from a 683 million euro bid for German 3D printing gear maker SLM Solutions after Elliott, which had a stake in SLM, said it would reject the bid. ($1 = 0.8464 euros) (Additional reporting by Tuomas Forsell in Helsinki and Maiya Keidan in London; Editing by Douglas Busvine and Jane Merriman)