Glencore International may need to double the premium it agreed to pay the owners of Xstrata, to complete the mining industry’s biggest takeover.

The firm, which owns 34% of Xstrata, offered 2.8 shares for each one of Xstrata’s. That ratio represents an 8% premium, but some shareholders say the merger undervalues their shares.

Yesterday, Xstrata’s biggest shareholders, Standard Life and Schroders, said they would vote against the merger if Glencore didn’t boost its all-stock agreement.

Only 16% of Xstrata’s register has to vote against the deal to block it.

Glencore can afford to offer at least three shares, increasing the takeover premium for Xstrata to 16%, reports Bloomberg.

Earlier this week, Glencore agreed to buy Xstrata in an all-stock deal currently valued at $37.6 bn.

The merger, if it goes ahead, will create a $90bn major natural resources group to be named Glencore Xstrata International, which is expected to have $209bn in sales.

The new entity will also become one of the five biggest suppliers of copper and nickel.