* Erdemir ordered to end partnership with ArcelorMittal
* Competition Board also fines Erdemir 20.12 million lira
(Updates with details, background)
ISTANBUL, June 19 (Reuters) - Erdemir, Turkey's largest steelmaker, said on Friday it was fined 20.12 million lira ($13 million) for violating competition rules and was ordered to end its partnership with ArcelorMittal.
Erdemir said in a statement it would appeal the decision by the government's Competition Board.
The Ankara-based board said Erdemir's purchase of a 25 percent stake in ArcelorMittal Ambalaj Celigi, a Turkish unit, was illegal.
It also ordered Erdemir to terminate its stake in Borcelik, a Turkish flat-steel producer, that is also owned by ArcelorMittal, the world's largest steelmaker, and Istanbul-based Borusan Holding.
Erdemir said in September it was ending its partnership with ArcelorMittal in Borcelik, a flat-steel producer.
Shares in Erdemir rose 3.3 percent to 4.36 lira on Friday. The statement came after the bourse closed. (Writing by Ayla Jean Yackley; Editing by David Jones)