Hydro said two members of Alumetal's supervisory board and all members of the management board had already agreed to the takeover proposal, giving the Norwegian company a stake of 39% so far.

"With the transaction, Hydro will strengthen its recycling position in Europe and widen its product offering in the low-carbon and scrap-based foundry alloy market," the Oslo-based company said in a statement.

The offer of 68.4 zloty per share represents a 4.7% premium over Thursday's closing price, but is still below last month's all-time high for Alumetal's stock of 77.8 zloty.

The Polish firm has an annual production capacity of 275,000 tons of aluminium from its plants, three of which are in Poland and one in Hungary, Hydro said.

The proposed transaction implies an enterprise value of approximately 1.33 billion zloty, based on Alumetal's latest reported debt and dividend plans, the Norwegian firm added.

($1 = 4.4514 zlotys)

(Reporting by Terje Solsvik; Editing by Jacqueline Wong and Sherry Jacob-Phillips)