* Elliott stake comes amid planned takeover by Fortum
* E.ON can sell its stake to Fortum for 22 euros/shr
* Uniper shares up 4.2 pct at record high (Recasts, adds confirmation, E.ON comment)
FRANKFURT, Dec 5 (Reuters) - German energy group Uniper said on Tuesday that activist investor Elliott, founded by Paul Singer, had taken a 5.32 percent stake in the company which is the target of a takeover bid by Finnish peer Fortum.
The stake comprises a voting equity stake of 2.39 percent, with a further 2.93 percent held through unspecified instruments, Uniper said in a regulatory filing, confirming an earlier report in the online edition of Manager Magazin.
The bet, worth 480 million euros ($568 million) based on Uniper's closing share price on Monday, marks Elliott's latest foray into Germany's M&A arena after the fund took a stake in drugmaker Stada and successfully extracted a sweetened offer for the group from private equity firms Bain Capital and Cinven.
Last month, Uniper rejected a 8.05 billion euro takeover bid from Fortum, saying it undervalued the company. Still, Uniper's former parent E.ON has the option to tender a 46.65 percent stake it still holds to Fortum for a fixed price of 22 euros per share by Jan. 11.
Uniper's shares were up 4.2 percent at 25.955 euros, their highest level since they were listed in September 2016.
E.ON, which spun off Uniper last year to focus on renewables, networks and power retail operations, said it would make a final decision on the deal in January. It may have to pay as much as 1.5 billion euros if it does not tender its stake.
($1 = 0.8449 euros) (Reporting by Christoph Steitz, Ludwig Burger; Editing by Douglas Busvine and Jane Merriman)