UPDATE 1-Turkey's Erdemir ordered to end Arcelor partnership

* 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 (EREGL.IS), 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 (ISPA.AS).

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)