At least two top 10 shareholders in miner Xstrata plan to vote against a takeover by commodities trader Glencore, threatening the creation of a powerhouse spanning mining, agriculture and trading.

Standard Life Investments and Schroders said yesterday the deal, the mining sector's biggest, to buy the remaining 66% of Xstrata for $41-billion, undervalued their shares.

The deal, designed to create a company to rival mining heavyweights such as BHP Billiton and Rio Tinto, needs to be approved by 75% of shareholders excluding Glencore, which is barred from voting.

Standard Life, the fourth-largest investor in Xstrata, and Schroders together own 3.6% of Xstrata, but 5.6% of the shares needed for approval, according to Thomson Reuters data. Their stand may persuade others to follow suit.

"I'm in complete agreement with Standard Life and we intend to do exactly the same. This is a fabulous deal for Glencore, it's probably a great deal for the Xstrata management, but it's a poor deal for Xstrata's majority shareholders," Schroders' Richard Buxton said.

Broker Liberum Capital said in a note: "Only 16% of Xstrata's register have to vote against the deal to block it, which means there is a significant risk Glencore's proposal isn't passed."

Xstrata CEO Mick Davis, who will be CEO of the enlarged company, admitted the two companies would have to work hard to bring some of his shareholders on board.

"We clearly have to now go to our shareholders and speak to them and take them through the transaction ... we've got a long gestation period, we recognise that," he told analysts.

Xstrata shareholders other than Glencore will hold 45% of the new company, to be named Glencore Xstrata International, even though Xstrata assets would comprise about 65% of the combined group's asset value.

Terms of the deal look set to spark tensions within investment houses, which hold both Xstrata and Glencore stock.

Portfolio managers at a third top 10 investor are due to discuss the merits of the tie-up and the relative value for investors in each firm before deciding whether to reject or back the deal, a spokesman said.

The new group, with mining assets from New Caledonia to the Democratic Republic of Congo, is expected to use its clout to look at other deals, including potentially a takeover of Anglo American.

"M&A is a space that you'd expect the combined group to be in," Davis said. "We have a combined entity which has much greater flexibility to be opportunistic and capture the right opportunities when they are there."

Anglo American CEO Cynthia Carroll declined to comment on the deal.